High idle

there are far too many variables in your situation to make even a good guess. If you have a mechanical accelerator, it could be sticking. Most newer cars have electronic accelerators, which do not have those problems. Most likely, something about the condition of the engine is causing the computer to keep the high- idle curcuit open. It could be many things, though. You need to see a good independent mechanic.

Vacuum leak, locate and fix.