Hi,
You can check the Flywheel positition sensor and the rpm sensor at the engine, when the flywheel position sensor is not able to send as signal to the ecu, the ecu do not know when it has to send the ignitionspark to the 1st cylinder,
the sensors are mounted,when you look from below the car in driving direction above the driveshaft on the left side under an guard plate, near the gearbox, when you remove the two bolts you can take out both sensors (thier mountet together on a braket). The electrical recistance must be approx. 1 k Ohm.

I hope i could help you.

Gude Mario Audi 100 CD Autom. Bj.83 Audi 200 20v Bj.89 Audi 5000 Bj.85 Audi V8 3,6 Autom. Bj.88