[tccrab]

Check for vacuum leaks.
I had a funky idle similar to what you describe after I changed my injectors last year. Turns out I had a small vacuum leak at the funky one piece two pipe PCV connector just behind the throttle body on the drivers side of the plenum. If it's not exactly in place it will leak and cause you all kinds of grief. GM realized it was a problem because they changed it to a different connector in the following years.
Use a portable propane bottle (the one the plumber uses to solder pipes), don't light it but crack the valve and spray around the hoses while the car is idling. Where's a vacuum leak the propane will be sucked in and the idle will increase a little. The bigger the change, the bigger the leak.
What you are looking for is a small leak that will temporarily seal when the engine gets warm or if the RPM's are up and the engine vacuum is greater.
Also check the PCV hose that runs behind the plenum, it's a common fault, it degrades from engine heat and cracks.

Good luck!!!