Well wether you think much of it or not, I have encountered this problem in many many V6 GM motors. And I didnt arrive at that off the top of my head, due to the fact that there was a recent $500.00 tune up done and there is still a problem lends validity to that as well.
After several years of fighting with these engines, I started to see a pattern develop with alternator ripple and ignition system issues. Like I said, it doesnt have anything to do with the alternator charging or not. The alternator is what supplies the 12 volts to run the car, when diodes or diode bridges fail, AC current is allowed to travel along the circuits and it creates havoc for the computerized engine controls. Invariably, the vehciles that developed this problem had tensioners that were original and had been on the cars for years. Usually you won't even know the tensioner is bad until you go to take the belt off and find that the pulley is positioned way off from where a new one would be. I have attended training specifically on the 3.1, 3.4,and 3.8 engines and have worked on literally hundreds of them-there is a whole chain of events that starts with the peice of crap tensioners GM puts on them.
The catalytic converter, if it is restricted, may not be resricted enough to create problems cold, but if excessive heat is building up in front of it, the harness and components that are located on the back side of the engine ( or engine compartment) get overheated and I have also seen this as an issue repeatedly.
You should also consider the possibility that you are having a fuel pump issue. If the pump is failing it will get worse as it runs longer, and that would also cause the sypmtoms you are describing.
Let me know what you find out. Thanks for using JA