I would check the timing,,,Theres a wire plug in you have to disconnect before you set the timing at 0deg TDC,,,,Its worth it to check the timing,,,these GM TBI motors are a pain in the ass to diagnose but when theyre running good they really do run good. High idle could be your timing is too far advanced.
But most common cause is vac leak, aftermarket parts like coil, pcv valve, whatever can cause big problems.
I had an msd coil on mine and it fried the rotor inside the cap, was a new cap and rotor, I had to take it to a mechanic to have him go through every component until he checked the rotor and found it was cooked inside. You couldn't tell it was bad by looking at it. Replaced it and the coil and its been running good since.
