car has trouble starting, what could it be? Options

[6strngs]

yeah, I've pretty much noticed this since a little before I got the car, but when I go to start it up after it's been sitting a few hours, it'll start, and the RPMs will shoot up to where they are supposed to, but when they come back down, they don't stop at 750 or whatever idle is and keep dropping until they reach zero and the car dies... so I'll start it back up, and in order to keep the RPMs up, I'll rev the engine. but as soon as I let go of the gas they drop real low again, so I rev again. usually, once I rev it to ~6000 and let it come back down it'll idle fine and then run fine for the rest of my trip, and if I shut it off for a while and come back, it'll be fine. but if it sits for 6 hours-ish or more before I start it up again it'll do it again...

what could be causing this?

[6strngs]

ok, I never did come back to update. I took what I believed to be my iac off and cleaned it, stuck it all back together, and it still does it. in fact, it's been getting worse lately. this morning it didn't even start until I gave it gas (normally it starts and then dies right away, this time it didn't start at all) and it's been doing it more often. are there any other possible things it could be?