Just guessing, I'd say the IAC, but if it's bad enough, I would hope the check engine light is on. You sure don't want to start changing parts without knowing for sure. That gets expensive.
O'Reilly Auto Parts
There are several malfunctions which can cause rough idle-but smooth during cruise and acceleration symptoms. Any one-cylinder misfire caused by ignition failure such as a spark plug, ignition wire or coil-over-plug will seem to be better at cruise. A vacuum leak at an intake manifold runner-to-head gasket will miss at idle only. Broken or collapsed motor mounts will make the engine feel rougher at idle only. An individual fuel injector that is plugged or is electronically frozen will do this. Start with pinning down which cylinder is the culprit. Remove each wire or each coil connector in turn, start the engine and determine which disconnection makes the LEAST difference in engine RPM-that will be the misfire. If they
are all about the same, check the mounts.
Chino Autotech Inc.