There are four typical areas to look into:

1. Hall sensor in distributor. The wires from the sensor to the connector
outside the dist have been known to fail and give intermittent service.
Sometimes with the engine running if you wiggle the connector on the outside it
starts running bad.

2. Air flow sensor. VW has a harness to fix electrical noise problems in this.
about $100. Popping the black cover and cleaning the contacts sometimes works
too.

3. O2 sensor wiring. If the wires to this sensor get grounded the brain gets
real weird.

4. Grounds. Check and clean every ground connection in the engine compartment
AND under the back seat where the brain lives. Make sure you check the separate
brown ground wire next to the brain.