I'd say it's the IAC but it's strange it hasn't thrown a code....tara do you hear any hissing under the hood?? vacuum leak maybe??? massive vacuum leak?? Sounds like a sticking IAC....is your car under warranty?? if so take to the dealer and tell em what's goin on. and welcome aboard as well.
I would venture a guess to a massive vaccum leak or stuck throttle blade or IAC. If the tps is out of range, the ECU should relearn this new value as the closed throttle voltage after you shut it off and restart it, then it would idle pretty close to normal.
I noticed that my idle would every now and then go up to around 2K. Later on i figured out it has something to do with my heater. The idle will be around 2K only when my heater is on. So when i was sitting at a light i turned off the heat and my idle dropped down below 1K and was completely normal. I turned the heater back on, and idle went back up.
So yeah, it hasnt caused any problems with the car at all and has never ever done it when the heater is off. I dunno if your high idle works the same way as mine, but just letting you know you arent the only one out there. Do you guys think i should get mine fixed right now? Will it even cause trouble in the future? Or is it alright to not fix it this second?