Idle speed problem
 

My 1995 Aurora developed a problem last summer that seems to be heat related. When the temperature is very warm to hot, around seventy or above, my car wants to stay at around 1100 rpm when I take my foot off the accelerator. The accelerator even jumps up and down (it can be heard) and the car sometimes surges. I have tried turning off the cruise control to see if that was the problem, but it had no effect. The dealer kept my car for two days last summer and could not duplicate the problem and there were no stored codes. I bought the car new and it has 263,000 miles and runs great, gets 25mpg on the highway, but this erratic behavior is scary. It is hard to drive the car in traffic. Anybody have any ideas?