First and foremost, welcome to the Pelican Porsche community and P-car ownership!
Second, and probably most important…when was the last maintenance done? Check the records which came with your car since they will provide you with very useful information about the care taken prior to your ownership.
You may find that the car just needs a little bit of adjustments, such as:
1) a tune-up
2) new components (spark plugs, air filter, fuel filter)
3) valve adjustment
4) Timing set
5) Oil change
By what I gathered from your information posted here, your idle should settle around 950 rpm when the car is warm. One thing to check is the distributor itself…does it move when you try to rotate it to the left or the right? If it does, this could be a problem and there is a nut that can be tightened up to secure the distributor from moving while driving. Just a thought.
Do you have Wayne's book 101 projects, or the Bentley Manual for the 911SC? These are good sources for information, as well as the Porsche Factory Manuals.
As mentioned above, the CDI system could be the culprit but I hope not since that is expensive to diagnose and replace. When you turn the key on, there should be a high-pitched whine coming from the engine compartment which indicates the CDI is working.
I do hope it is something basic and not something major which provides the fix.
Good luck with your investigations and have fun with your car.
