GM-GMC: idleing high, intake air temp, vacuum leaks

Question
I have a 97 chevy p/u and lately it has been idling high no matter if you start it hot or cold then it gradually drops back to normal. and then when you stop and shift it in to neutral or park the rpm's jump to about 1800 then gradually drop back down. I have changed the TPS and the idle air control valve with no luck. hopefully you can help...thanks

Answer
The most obvious thing that you could check would be for vacuum leaks, that would cause an elevated idle with relatively no other drivability concerns.  Aside from that the next step for something like that would be to hook up a scanner and look at some of the values that the various sensors are reading.  It could be an Intake Air Temp sensor problem or your TPS could be reading slightly high etc etc.  Have a look around for a vacuum leak and see what you find.
Hope this helps.