2001 Regal 3800 69k miles. When cold, engine starts well, but unstable idle speed. Let it idle for about 1-2 minutes and the idle speed starts to swing btwn 500 and 1500 rpm. Eventually swings too low and stalls. After the stall, it will restart and runs normally.
Has new fuel filter, air filter, catalytic converter, O2 sensor, good fuel pressure, no vacuum leaks that can be found. No check engine light coming on.

Try cleaning the IAC (idle air control) valve with throttle body spray cleaner. Clean the IAC pintle and the seat where it attaches to the throttle body. Located at the top left hand side of throttle body. Another possibility could be the MAF sensor. Also, you can check the TPS (throttle position sensor) output to the PCM module to see if in specs. Around .4 volts at idle to 4.5 volts at WOT.

After removing and cleaning the idle air control valve, the car did not improve. I waited for 2 or 3 tanks of gas to go through, and now it runs perfect. Maybe I got some bad gas?

Note to others - the IAC valve is held in by 2 small (m5 I think they were) torx screws. The factory used LocTite on these screws, and one broke taking it out. It is hard to judge torque on a small screw like this, and very easy to twist it off. I got allen head replacements at Lowe's (note they are the coarse thread version) and a tap at Sears; drilled out the bad screw and tapped the new hole. A lot of time and expense could have been saved with a lock washer rather than LocTite. Apply heat (carefully) before trying to remove them.