As track mr2 says .  Very unlikely to be ECU.   Try replacing the coil first .  Coils have a tendency to crack their outer casing.  Particularly as they get old. 

It could be the ECU. I had similar symptoms.
 
Mine went about 4 years back. One of the capacitors inside were leaking. Symptoms were the engine would just turn off while driving, independent of anything else (speed, load, rpm, boost etc). The speed and tach would drop to zero and the engine would stop. Because the ignition was set to "on" when the engine shut off, the dash lights would be active - I never lost power (ie. radio, lights etc.) but the engine stops because there is no spark/fuel.
 
After sitting overnight and having a cold start, the first time it would turn off 20-40 minutes after driving, then 10, then 5. It would progressively decrease that time until it would just crank over and not start. ie the more "warmed up" the car was, the more prone it was to shutting off.
 
ECU's are fairly easy to find. I'm sure there's a JDM wrecker near you. Otherwise, I know that Shine Auto in Underwood here in Brisbane are wrecking 2 SW20's at the moment. Also, you could try www.mr2wreckersaustralia.com.au 

thanks guys for the support
 
jsnhrl
 
that does sounds like my symptoms I experienced, although I just drove it only for one night,
the drive-able time will progressively decrease as well,
 
I'm currently hunting for a 93 turbo JDM ECU
 
will contact all the wreckers and mr2 specialists which everyone has kindly provided me with
cheers
 
 
 

Hmm, think the test drive should have rang some alarm bells. 
 
Hope you didn't pay too much for it initially, looks like its about to cost you a bit now. 
 
Paul Brell is the NSW/Sydney MR2 specialist. Save time and money with misdiagnosis and guess work and get it over to him I suggest.
 
If there's a problem with these cars, he's seen it many times before.

You should get the part number off the ecu to ensure you buy the correct one.

I have doubts on it being the ecu as well.

Paul is actually not too far away in Girraween If your car is in Parramatta. You could get your car towed there and get a more accurate diagnosis.

Crack open your ECU first and have a look at the capacitors/general condition of it. Might save you a few dollars rather than buying random parts on a hunch.

If you know someone with the same car and is willing to swap ECU to test, it would be the easiest way to confirm if it is /isn't the ECU. It's a few minutes to swap them.

I did this with the AFM on a mates car, had random power loss issues. Suspected the AFM, we did the swap and it confirmed it.

Cheers

Yeah, I've had numerous non Mr2 mechanics suggest the ECU is at fault... only to later find out they have all been perfectly fine.  PLEASE do as others have advised, have a reputable specialist look at it.
 

Hmmm this sounds oddly familiar...

Couple of years ago, a local guy with a Bathurst MR2 had an issue where the car would just randomly cut out and refuse to start.  Sometimes for a few seconds, other times, minutes.
 
Ended up finding that the cooling fan ECU in the boot next to the ECU was playing silly buggers.  I plugged his one into my car, and suddenly, I was unable to turn my car off! Only way to stop it running was unplugging the suspect cooling fan ECU.  It was one of the most oddball issues i'd ever seen on these cars!
 
Anyway, we got the guy a replacement fan ECU, plugged it in, his car ran fine and I can only assume its still going well to this day.