I have had my '83 Ur Quattro for about 10 years and have replaced/ rebuiltmuch of the engine and related components. Recently, the car's idle will occationally drop froma "smooth" 1000rpm to a very rough 500 rpm and the subsequent"performance" degrades significantly. (It will barely accelerate). It will not clear itself after this happens yet is just fine the next day so it appers it may be temperature related. I was even wondering if it could be a basic component like the ECU/chip itself. I know this is a "vague" description but just in case any of you may have had similar issues, I would appreciate any feedback.

 

what about vacum leaks ... then temp sensors .. and the ISV (temp sensor dependant) you can clean it for good mesure

 

Try testing the idle switch they can work intermittantly.