Ok, this car is driving me nuts. Every once in awhile the engine will rev up while your foot is off the pedal. It has slowly been getting worse, tonight revving to about 3500rpm. So you let off the gas and the car continues to accelerate. Kinda hairy if you're trying to brake and the engine is fighting you to a stop. The SL1 is being a bad car. Any ideas? Thought it was the IAC, replaced that. We pulled the cruise fuses tonight to verify it was not the cruise control trying to engage. Will drive it to/from work tomorrow to see if it is cruise control issue. Thinking about looking for vacuum leak tomorrow in the daylight. Would a fault with the air pump system cause a high idle issue? What about a fault with a transmission solenoid?
I would check the vacuum hoses to see if something is wrong. Also, the EGR valve is a common failure point on these cars. It can stick open, and cause the symptoms you described. If the EGR valve is gunked up, you can try cleaning it out with carb cleaner, and see if that helps.
The AIR system just pumps air into the exhaust. So it shouldn't affect the running of the engine. The transmission solenoid shouldn't affect the idle like that, either.