Alright well, time to spit out my thoughts.
I've had the same prob since I've replaced my engine. And the possibilities are a combo of pretty much everyone, and then some
I have been told-
-Adjust Idle screw on Throttle Body
-Spray Carb Cleaner on all vacuum lines and check for leaks
-Bad MAP sensor
-Throttle Positioning Sensor
-Clean Your IAC Valve (really easy)
-Possibly air in coolant lines
Now, I'm not recommending you try those in that order. Try the cheapest first, start from the bottom and work you're way up. Sometimes it can get expensive, though half that needs to be done anyways after 100,000 miles anyways just to keep it runnin like a champ.
Just remember, if the gaskets on these sensors break (TPS) than it WILL mess up, because even the smallest amount of air/moisture will mess it up. And MAP sensors, just tend to go bad after so long.