Vac leak is the most likely culprit but there are a coupled of other possibilities.

I assume we are talking about idle so the IAC could be sticking...usually a good cleaning will fix it.

If the TPS switch is faulty or not engaging correctly because someone tried to adjust the idle with the stop screw. This usually happens when the throttle-body has become dirty enough that the butterfly does not seal/seat correctly at idle so a bubba not familiar with the system adjusts the only thing they can adjust on an LH 2.4 system.

In short, clean the throttle-body to include the vac line fittings, clean the IAC, put it all back together and check for vac leaks. While you are at it clean the flame trap and take the accordion tube from the AMM to the throttle body off the car to check for spots that have been worn threw or cracks that only open up occasionally. The total cost in parts is about $7 for the TB gasket and can of TB cleaner. Total time should be about 1.5 hours if you have not done it before.

Mike