Pontiac: Rough idle, pontiac grand prix, grand prix 3

Question
2000 pontiac grand prix 3.8L K engine.

After car is warm and you turn off and let it sit for about 30 minutes then start the car the idle will be normal for about 30 seconds then will start to get rough. It only lasts about 20 -30 seconds then is OK. It acts like it is trying to go into closed loop before the O2 is hot.  Voltage at O2 when started is 1.3VDC and goes down as it warms up. Had it in 3 times and no one can fix.

Answer
It sounds like it could be a coil pack or an injector that is being affected by the heat of the hot soak(sitting for 30 min.) Since the problem only lasts 20-30 seconds, it is going to be very difficult to pinpoint the cause. It may have to get worse before it can be diagnosed and repaired.
I know that you are probably frustrated with it by now, but if it was a serious problem, the "service engine soon" light would be on, which would help to diagnose the problem.
Are you taking it to the dealer for this problem? If not, you should. We have all of the proper equipment to correctly diagnose the problem.