Sounds like a throttle body valve plate sticking.
Yeah who knows. I actually removed and disassembled the idle air control valve twice, spraying it down with whatever the parts cleaner stuff was that said it's safe for that type of part. A little bit of black carbon stuff came off, but otherwise it was actually very clean already. I also sprayed a bunch of parts cleaner all over the throttlebody itself. The IAT and coolant temperature sensors appear to be good and reporting accurate numbers based on my OBD-II device. The TPS returns good value, as does the MAF. Oh yeah, cleaned and inspected the MAF too. Fuel economy is good, which generally rules out a dramatic problem with the MAF or O2 sensors. Replaced the spark plugs and wires, and verified the gap on the plugs before installing them. Old plugs actually looked really good, which generally rules out an ignition problem. It's tough, because everything I checked always looked good, and any time I replaced parts it ran the same.
Like I said though, at least with my car the problem went away when the temperatures dropped, so I'll just wait and see how it goes over time this spring as it starts getting warmer. I still have to get an estimate for having the A/C fixed. If that's affordable then I'll probably do it, but if the A/C is going to be $1500 or something, then I'm just going to ditch the car entirely and get something newer.