I'd check for error codes first before doing anything.

New here and would confirm ecu. Just clean the board. See if improves. If Improves replace capacitors.

So finally back from holidays. I have replaced my o2 sensor and my idle revs have Stabilised. So I took it for a test drive today. It stalled again. However before I ran out of speed it kicked back in again and drove along. It’s the first time it’s ever done this. Are we still thinking ecu? I might have to get the ecu pulled out on the weekend

anyone with ECU's (stock or aftermarket) which are older than 10yrs should consider replacing all the electrolytic caps. Electros are reputed to wander out of spec (or fail) after about 7yrs.  For me it was quick as I have a solder sucker, and a Camry V6 2001, amazing that almost all the cap values were the same as my 90 SW20T.  Both cars responded nicely to new caps.
 Re cutting out at idle, I remember leaving one of the air hoses off while I was testing something. Car would not idle at all, until I put my hand over the opening. 

TwoDogs
anyone with ECU's (stock or aftermarket) which are older than 10yrs should consider replacing all the electrolytic caps. Electros are reputed to wander out of spec (or fail) after about 7yrs.  For me it was quick as I have a solder sucker, and a Camry V6 2001, amazing that almost all the cap values were the same as my 90 SW20T.  Both cars responded nicely to new caps.
 Re cutting out at idle, I remember leaving one of the air hoses off while I was testing something. Car would not idle at all, until I put my hand over the opening. 

My idle problem seems to be solved with the replacing of the O2 sensor. Now its just damned stalling problem. Which happens at all different speeds but it seems only once the car is @ operating temp. As I said, it stalled yesterday, but then as I was slowing down it came back to life with a jolt. 

Trouble is it could be several things - bad sensor, or sensor connections, 
Just some background - how long does it take to come up to "normal" temp? (needle at 9 o'clock). Mine takes 2-3 mins of sedate driving.
Have you set/checked the ignition timing? 
Fuel supply is unlikely to random, but more about the greater the demand, the greater the problem.
Loose connections, and as someone suggested, the low tension wires feedign the distributor...mine had cracked insulation and were shorting out their 5v intermittently. Once spotted, cleaned and taped up - problem gone.
Perhaps even a cracked lead to a sparkplug arcing to the engine might be enough to stop the engine with no throttle ?? Oil seal in the distributor !!  Quite a few things to work through...

Since I had rebuilt it I replaced all the leads etc. Im thinking there could be a fault in my distributor as I never replaced it & it looks pretty horrible tbh. My ECU is an uncommon one according to Josh @ the MR2 Wreckers and is quite expensive. 

Maybe I just ditch the Dizzy and go a microtech with coils  
http://microtechefi.com/product/lt-9c-with-x4-coils-package/

Alright so I pulled the ECU today and these is what I found.

Firstly it smelt really bad when I opened it up. Dont know if this is usually common but yeah.
 
- On of the capacitors had some black residue at the bottom which ive attached a photo of. 
- The board itself looked quite clean with no obvious signs of burning or major damage
- The case had a slight dint in it, which on closer inspection had black dust on the inside. Maybe it was touching the board at points?
 
Other than that the board seemed quite clean. I've attached some more photos - if any of you guys could have a look and see if theres anything glaring that ive missed?
 
Jarrad

The capacitor in the first photo is shot. As some one else said look for other caps that look swollen and replace them.
Its a easy job to replace them if you have a solder sucker.  I am in Sydney if you need any help.

kojab
The capacitor in the first photo is shot. As some one else said look for other caps that look swollen and replace them.
Its a easy job to replace them if you have a solder sucker.  I am in Sydney if you need any help.

 
Yeah I live around 4hrs west of Sydney. I'm not real confident with electronics, so id rather get someone who knwos what there doing to do it.