Interesting, as my '89 80 2.0E does the same thing (oscillating) when warm, although not all the time. I've never looked into the problem though, as it's not at the top of the list of issues my car has.

Look forward to seeing the solution/outcome though.

 

It is important to have a factory (Bentley) service manual when working on Audi's but don’t get your hopes up too high. The manual sucks! At least for the 100/200 models, just sucks a lot less then Haynes or Chilton's.

I heard that the ECU for the 80/90 models is located under passenger side of dash.

The voltage regulator problem on my car is minor inconvenience during the first few seconds of operation eventually I will get to it. I don't think that shaking has anything to do with it as one common denominator is that voltmeter has to dip down to about 10-11 volts. It seems that the regulator has got to go under load before it starts to regulate voltage. It also seems a lot worse if the car sits for a few days and the battery doesn’t quite have that full charge.

 

Well, replaced ECU, no effect, (got it free anyway) then replaced ISV (again, Free) THIS fixed the problem. Thanks to all for their assistance.

 

Thanks for letting us know. Often we don't hear from people and don't know if they were frustrated and torched the car or solved the problem.