Idle problem after car was overheated

[rknorr]

1989 cavalier 2.0 liter: car overheated due to hole in heater hose, hoses were replaced and car test driven. Car was hard to start and missed at low rpms, I pulled the sparkplugs and found one of the plugs had a broken insulator so I replaced that one. Car now starts easier and runs well but now car takes several seconds to return to idle after the accelerator has been released. I lubed the accelerator cable but that did not help. I checked the linkage at the throttle body and the throttle appears to return to the idle stop screw immediately after lifting off the accelerator yet the car continues too run fast, I'm guessing 1500 rpm, for a few seconds before returning to a normal idle speed. This is a little disconcerting when trying to stop, the engine is continuing to drive the car while you are trying to stop. One more detail; the car did not do this prior to being overheated. Any ideas?