Sounds like the power brake booster is bad.
no .... no way.
the idle jet in the carb is blocked and needs to be cleaned out and reset
Most likely the ignition module.
Idle Air Control Valve.
check air intake hose for cracks between the ribs causeing to much air. if ok check dist rotor for burnt marks. also you should check brake power booster when appling the brake and this uit is leaking it will drop idle speed.the brake pedal should fill hard.
My best guess is that the Idle Air Control (IAC) valve may need to be replaced.
Idle Air Control Valve would be a good place to start.
belt is loose.
throttle positioning sensor, egr, idle air control
Check for a major vacuum leak.
sometimes u need to let it idle for a few. then put it in gear but keep your foot on the brake. see how much your idle fluctuates. mines got the same problem. i think it could be the throttle positioning sensor but im not positive.
it sounds like your tcc (tourqe converter clutch) switch is faulty. this is a common gm problem.