So replaced the water pump over.the weekend all good. went for a test run... started to heat up... so i.got a new thermostat... pulled.out the.old to replace.. and well they gave me the wrong thermostat... joy.... boiled.some water to test my.old.one as it even looks fairly.new... worked (didnt get to test proper temp as no thermometer) drilled out the bleeder hole a.touch.larger... and reinstalled... rebleed... test run... all.good. gave it.a.bit.of a workout for some time. all seemed fine... went for a drive cross brisbane.. all good until 2nd light from where i was headed... started to heat up.. threw the heater on full blast.. temp came back.down.... quickly... got to.next light... up she.jumped again...

Thoughts? was thinking maybe the thermostat is sticky.. and every once in a wjile gets stuck needing more pressure from more heat to pop the seal...

so bout the full stops... typing on phone on the side of the road. waiting for her to.cool.down some.

Update .... couldn't be thermostat, as while driving / cruising ... it will stay constant, normal operating temperature, only heats up while idle, pulled the plug on the Thermo-fan switch leaving them on as a constant to get me home.
 
Starting to fear the worst and think it may be a warped head, turbo spools, push's air into the cooling system ... after a while it would create enough of a bubble to heat.
 
Anyone got any tricks to test this before I decide to pull the head off?...

Do the thermo fans work? With the switch plugged in went through this myself,

Fans wouldn't turn on, bled the system properly, fixed my problem
Also fixed my overheating issue

Test thermostat
Take the thermostat out and see if that helps

Check if you have warped the head
Do a comp test any that are dramatically out will usually be two cylinder that are next to each other
Will tell you if the head gasket had blown or you have warped it, also of there is water in your oil?
Milky colour head gasket/head issue

Is it the correct water pump? Etc Alot of the aftermarket ones are junk

But sounds to me you have air in the system
Because if you had blown the head you would defo know you had, by now

Had several cars before that have blown heads etc and no water in oil etc, the water pump I used was a GMC, same that came out.
 
Bleeding of the system may be overlooking something?... did seem a touch odd with the 2 bleeder hoses up they did not seem to be at the same level ... close though... also, ever so slowly it does seem to drink the water.. so air in the system may be the key ..
 
I have tried the 3 minute idle with both bleeders shut off, and both open, both times with radiator on one click.
 
Have also tried filling up the filler cap .. and releasing water from the bottom of the radiator recap, top up and open bleeder again (to pull excess air from the lines).
 
The other thing is that it seems to take a little time before it starts happening, like 20mins or so..

It might sound odd but I would try bypassing the heater pipes. The heater pipes are prone to crack above the fuel tank and I swear this was causing my water temp issues, whether it was slowly pushing water out there or sucking air in and would never let the system to be bled properly. Just connect the heater entry and exit together at the rear of the engine with one of the pipes to eliminate it for the time being. 
Worth a shot I think. 

Could be worth a look into, but I am sure I would at least see drips while either at idle, or right after it was turned off, I've watched it meticulously during the "test" I performed, may try that simply to eliminate that as a problem though, cheers.
 

very often this is caused by air in the cooling system. Open the bleed screw on the radiator and check that water is coming out, then retighten, then open the bleed screw above the gearbox and check for water coming out again, then retighten.

"the water pump I used was a GMC, same that came out".
What is a GMC???
Some aftermarket pump inserts have smaller vanes than the OEM ones so it is always recommended to use genuine Toyota replacements.
 
"ever so slowly it does seem to drink the water, so air in the system may be the key".It does seem suggest that air is still in the system.
Patience is needed with MR2's and it is not uncommon to have to top up for a week or so after refilling the system.
 
"did seem a touch odd with the 2 bleeder hoses up they did not seem to be at the same level"..
I have found that if air is still in the heater, the level can be lower in it's bleeder hose until it is purged out.
 
BTW, have you tried feeling the pipes in the frunk to monitor the coolant temp in and out of the radiator. Could tell you if the coolant is circulating ok.
 
 

Gatesys SW20
Do the thermo fans work?

^This!
Check the thermofans first. If the overheating only happens when you're stationary, it is most likely a problem with the thermofans.

do a pressure test on the cooling system. That way you will find out hidden leaks. Mine only leaked after 10mins of testing and it was very little. Turned out to be a cracked block near the waterpump area