1990 911 C4 Problem

I just purchased a 1990 911 C4 with 66k miles on it. It had some work done to the oil lines before I bought it to fix some leaks etc. Overall the car runs really well with one exception. When I'm sitting at a stop light for more than a minute or two the idle drops and the car will seem to lose power and stall. The only way around this that I've found is to continue to keep the engine revving around 2000 rpm? Help please.