It sounds like the APPS (Accelerator Pedal Position Sensor) and it is probably going out. You can search the 2nd gen forum and the 911 archives. Several members have taken them apart, cleaned them, and put them back together and they worked like new. Be aware, they are not intended to be taken apart and cleaned, so you will have to cut the rivets out to get it apart. Or you can plop down 400 big ones for a new one. This is covered under the 5 year / 100,000 miles warranty if you are not out yet. It will probably continue to get worse until something is done. Good luck.