I agree with len, the ecu is providing fuel and spark so if the timing belt slipped during start-up, or the hall sensor has a broken wire thats intermittent or is just dodgy, that could be the problem tho its strange no dtc's were thrown up. Jeez most mk3's throw a camshaft speed sensor error 000515 just cause you scan wi the engine not running. Did you see the output on vag-com, i.e. is the ecu communicating all the right info everywhere else?
check the cambelt timing. I had an audi 80 which the belt slipped on, musta happened during start up as it fired up ok and was warming up in the driveway, then it cut out and wouldnt restart. In desperation after weeks of tinkering, i decided to check this as i had done everything else. It was three or four teeth out and the tensioner was seized. I think it had heated the belt in that first few minutes and caused it to soften, stretch and jump. I sorted it, new belt etc and away it went like nothing had happened!
Remember that if you clean the TB or remove it or the accelerator cable then you must do a TB alignment with vag-com, its very easy to do. That can stop the car starting if not done.
Maybe the dizzy has slipped round or the wee carbon brush in the cap may be cracked or sticking - that causes similar problems to yours.
Or you dropped your key in a puddle and the chip is dead.....
