I have a 98 sohc atx and my problem is with my car idleing down. Whenever I start my car I'll let it idle for a cpl seconds and than step on the gas a little, it used to idle back down just fine but now it'll idle down a little bit,stay there for a cpl seconds and than idle down all the way, this is'nt normal for my car. Now me and my family went on a trip to San Diego last weekend (a cpl hundred miles south from us) and the car was fine and I noticed this issue right when we got home. What can be the problem? I have a 3.0 cai which I cleaned the filter, MPX UDP and a Pacesetter header which one of the flange bolts has snapped off so I've tightened the remaining bolt until I get a new header next month, I don't if any of these mods have anything to do with it.
I've also done the oil and tranny fluid and gasket last month just for this trip.

 

Things that come to my mind are spark plugs, IAC, and TPS and possibly PCV valve. Hope this helps!