2000 Audi A4 Quattro, 1.8T has been at the local European Car Shop for two weeks and they cannot figure out it's ailment.

The car has been starting fine and running. But intermittently it will suddenly stall and not start again. You can crank it over as much as you want, but it won't even chug or sputter. Come back an hour later or a day later and it starts up. It may even start up 10 times. Turn it off and come back 10 minutes or ten hours later later and it won't start. No way to tell when and why it does this.
We have scanned it with an ODBII and no codes were reported.
The local shop tried the Fuel Pump relay and thought that would be it. But they ended up towing it back in a day later because the symptoms returned.
The local shop claims that the Fuel Pump is functioning (apparently even when it won't start).
I am thinking ignition module, MAF? But the local shop doesn't think so.
Any suggestions to look for?
Thanks.

2000 A4 1.8T QSM, B6 17" Wheels, Black Ebony, GIAC Chip, Neuspeed Exhaust w/Downpipe, ByPass Valve, RS4 Grille