1982 Porsche 911 issues

I have a 1982 911 that has a demon that my local mechanic can’t seem to resolve. When it is started, while still cold, the RPM’s vacillate from 900 to 1600 until it warms up. This sometimes accompanied by a failure to start after the car has been warmed up. Further, when I attempt to start the vehicle I sometimes hear a “pop” in the motor coming from the motor. Invariably the car won’t start again until it is thoroughly cooled down. Does anyone have a solution to my problem? Who is the foremost Porsche mechanic near Michigan?