I had a similar issue on a car once. It was the head gasket that was gone. Cylinders would start to boil the coolant through the warn head gasket. Sometimes it would be alright especially on the move but once you stop or go on a  really long drive it would start again. Pressure test will be able to tell you.

Might grab a few bits n pieces tomorrow to make a pressure "Plug" for the filler cap, anyone know a good pressure to throw into it?... don't exactly want to bust any pipes / radiator cores. ... leave it plugged up over an hour or so and see if there's any change...
 
All good ideas though, Hoping there is no major damage, but all was working pretty well just before I changed the water pump.
 
#Redtarga, there shouldn't be a problem with the GMC water pump as it was the exact same one that I pulled out to replace it with, and was working 100% before, only changed it because the inner seal wore out. Started leaking through the tell-tale hole.
 
I have checked the Radiator lines, before removing the thermostat on a test run after changing the water pump, it did get hot with radiator lines staying stone cold (down the road) heated up ... then quickly went back to normal, got home, and radiator lines where warm (operating temp)
This was one of the first things I checked as soon as I pulled up, and yes they where warm. 
 
I am thinking there may still be a few pockets of air, possibly gathering in the return radiator tank preventing the thermo switch from being activated, and leaving the fans to stay off. Have had this happen not too long ago, rebleed and all was good again.... fun fun times...

Ok so I'm assuming you know how to bleed it properly (I got annoyed when ppl kept telling me it was air pockets when it wasn't lol cos I bleed it dozens of times)
 
Just to clarify, does the temp needle rise up slowly or is it erratic?
 
"The other thing is that it seems to take a little time before it starts happening, like 20mins or so.."
- Do you mean the temp gauge?
At one stage my engine would overheat 65% of the time from cold start up, but after I turn it on and flog it the 2nd time it would be normal.
 
Hey Ethan there doesn't need to be water mixed with oil or coolant for the head gasket to go. At a very very early stage there would be bubbles coming out of the filler neck slowly so even then it might be hard to tell. Last thing you wanna do is take it apart :S
Luckily before I bit the bullet and decided to take it apart, my problem was actually faulty wiring that made the temp needle spastic. 3 Track days confirm that hehe.
 
I believe the system pressure is 13 psi

I would make sure your thermo fans work properly

Because mine stopped working and it would overheat

This was the turbo one
So what I did checked relays then the switch all had power

So I decided to bleed the system did it for a solid 20 - 30 mins

Both clear hoses up front on the bleeders held above the filler neck so the air rises
Pressure cap off
Idle till warm

Continue topping up filler to top of neck
Water in the hoses should not rise above filler neck

Lots of bubbles from filler neck and clear lines

Close bleeders pressure cap on first click
Held at 2-3000 rpm whilst brother rocked the car
For a few minutes opened bleeders
Lots of air and bubbles
Opened pressure cap continued topping up

And rinse and repeat

Towards the end my fans started working again (huzzah)

Not an issue since
Knew it wasn't a head or gasket issue as my comp readings were spot on

But I also was boiling coolant

I will almost gurantee it's air in the system

Also not sure if yours is turbo or not
But if it is
May well be the hose from hell

But other then that could be a crap water pump or thermostat
Any toyota should keep thermostats on the shelf
As there the same used in many other toyotas

And also tree is right aswell I was just saying in a severe case

The head being warped I highly doubt.

Start with the easy stuff
Never heard if a bleeder near the gearbox?
I thought that was a drain point, I thought it had 3 bleed points
2 up front
Then the pressure cap/filler neck

Good luck :)

No the temp is not erratic, it does rise rather slowly when it does rise.
 
All the information I have read including the toyota 3sge manual would be right in saying that both the bleeder hoses are to be raised above the filler neck, while car is on level ground. after filling to the point it won't take more, you replace the cap to the first click and start the engine until it reaches operating temperature (so the thermostat opens) and give it another couple of minutes of idle, to ensure most or all air bubbles have been circulated through to the highest points in the cooling system (IE radiator bleeder hose, heater bleeder hose, and filler cap). Would I be correct in saying this is the right way to bleed one?
 
That is a slightly different way to what I have read, but I am going to get stuck back into it tomorrow, so I will also try this way you suggest. Thanks Gatesys.
 
The "hose from hell"? I have seen a post about this, but wasn't after that information so skimmed past it, are you referring to water cooling hoses to the turbo, I have turbo and non turbo, the one giving me troubles at the moment is a turbo. REPCO did have a thermostat, however it was miles too large at the outer rim / "lip", so instead I opted to boil my old one as it even looks rather new, and opened the bleeder holes a touch.

Oh and also I would advise against a pressure test in the system
Mainly due to the fact it could cause a lot more damage

A lot if radiator shops do it
To open leaks up more so you have to buy a new water pump radiator etc when you really did not need it.

Just a thiught

I had the pressure cap off most of the time as a lot of my bubbles came out of there only out it on for the high idle and revving part

Gatesys by disconnecting the "plug" for the thermo switch (driver side radiator tank) that should keep the fans on 100% with ignition on. I found this info handy.

Yeah I did that aswell to check to see if my fan motors worked and once I knew that it wasn't the relay or the fan it was something to do with the switch

Seems the system in my case needed a good bleed now the fans come up once at operating temp am turn off and turn on etc etc

Update .... spent the whole day bleeding the system. and just to make 100% sure I did both methods, idle for several mins etc etc .... went for a test... all was good ... boosted up a few hills as a minor strain test ... idled fine etc etc no jump in temp .... ok time for more strain... went and "cut sick" for probably all of 30 seconds in the form of a couple of doughnuts to get the turbo working and strain the engine some... got down the road ... temp jumped again... took extra notice that thermo's did not come on... jammed on the heater full blast as I slowly accelerated (slowly to prevent boosting) temp came back down slowly at first then from 3/4 to 1/2 very rapidly, also noticed a fluctuation of heat in the heater itself... only water to escape was from the overflow bottle... heater on whole way home (not far) and temp stayed normal, but jumped up higher as I got closer to home.
 
Surely with the amount of coolant in the system not having the thermo's kick in wouldn't make the temp rise extremely quickly.
 
Unsure what to think now, as it only seems to happen under fairly high boost (11lb) I am leaning toward a slight crack in the head. On top of a buggered thermo-fan switch, and with the fluctuation in temp for the heater, I have not noticed if the heater is mixed in with the thermostat cycle, or if the change in temp is just due to bubbles as the coolant was boiling.
 
Might just park it for the mean time until I get a new switch, and thermostat.
 
Will also try a compression test in the meantime.
 
And cross my fingers it has not done a head.