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...

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

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.

W34PN
Don't worry about replacing it yet.

Open both sides fold the smaller board back and inspect the other capacitors.

Clean the board with isopropil(?) Alcohol but I used nail polish remover to get rid of the black ****.

Put back in car. If no idle problems then you have your issue. I'd say you've got a few more with larger black marks on board.


Once you have identified it's the issue then get them replaced. It's fairly easy but can pay someone also.

My idle problems have been fixed since I replaced my fauly O2 sensor. Now its just the stalling when it gets warm.

I just googled MR2 C810 and bingo.
If you search also for the ECU caps in the MR2, there are hits which list the dozen or so.
These can be ordered from Element14, or RS or.... for a few bucks. Then take the board to some electronics guy in your area and ask him to replace all the electrolytics. He will clean the boards and make any other observations.
Google will have everything you need.

Reddtarga
Anyone with suspected ECU capacitor problems should check out: (Especially the last page)
https://www.mr2australia.com/mr2play/tm.aspx?m=138518&mpage=8

 Thanks ReddTarga, I have a known good ECU coming for testing. If all goes well Ill try and contact Michael from that thread. Unfortunately all his websites are gone now, so he might be out of the game.

The bleed system (TVSV) which enables the 3sgte to boost beyond the wastegate spring pressure (7 psi) is turned on by the ECU. Commonly, not boosting properly is not the ECU, as other things can lose boost pressure, but since all you did was swap the ECUs, then I would put the old one back, just to see if the boost changes. This other ECU which has been supplied, was that car boosting ok ? You may end up just getting your old ECU recapped.