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]

tomorrow morning will be the ultimate test. if it starts without me having to rev it, then I was successful in my cleaning of the iscv. However, I never actually took the iscv off... I tried... but the four screws holding it on were really ****in tight! and in the process of trying to remove them, I stripped all the heads so I cleaned what I could using engine degreaser and Q-tips.... it really cleaned alot of crap out. and my car seems like it starts better. like, instead of the rpms jumping to 1500, then dropping to 4-500, then raising back up to 750, they start at like 1500, then raise to 2000, then slowly drop back down to 750, the way they should. But like I said, tomorrow will be the ultimate test.