My 1989 740 GL non-turbo has a rough idle...when it's sitting at idle (at a traffic light, for instance) the engine surges. The normal idle is at 1000 RPM, but when it surges it drops down to about 700 RPM and back to 1000. Every once and a while it will drop so low that the car stalls out. Could this be caused by a bad fuel pressure reg., clogged injectors, sparks...?

Also, the car will usually start right up, but sometimes I need to let it crank a little longer and put my foot on the gas before it starts. This usually happens when I try to start it after it's been sitting after being driven for a while. I'm gonna change the fuel filter for starters...could this also be a symptom of bad injectors?

The car seems to be running rich (black tailpipe and the inside of the catalytic converter was really black)...I'm gonna replace the O2 sensor. Any thoughts?

Oh yeah, the car has 94,000 miles and it runs great at speed.