1996 G3 2.0 16v "WON'T START"

Hi All, I have a Golf mk3 GTI 2.0 16v, I was on the way to work one day a couple of weeks back, and the car lost power, so I pulled over and tried to start,which it did, but then cut out and hasn't started since.
Called mechanic who ran tests as there was no spark and then he
started the illimination process, firstly changing the coil, then distributer and cap, then the crank speed sensor, still to no purring of engine.
Managed to get a second hand (working ok) ECU which got us the spark back, but now the car sounds like it wants to fire but doesn't.
There is a great spark from the coil to the dissy,but after a couple of rotations of the engine the spark goes! Can anyone HELP?


P.S. Do you have any ideas Boschmann or Anyone?

I'd make sure all the timing marks line up. I had one that sheared the pin that holds the crank timing sprocket to the crank. Most of the time it turned with the crank & spark was close so it would almost start, then crap out. I finally caught it not sparking for a few seconds & found timing belt marks were off. You can try turning by the cam sprocket & watch the flywheel closely for any slippage

I'll ask the mechainc to check those points, Thanks