Chevrolet Repair: voltage problem, pontiac bonneville, wiggle test

Question
Hi, I have a 2002 Pontiac Bonneville SE w/ 3800 V6. Its not a chevy but still GM product so I thought I'd try and ask anyway. I get off work at 3am when its really cold and on the way home the dash lights/ headlights dim and the voltage gage on the instrument panel goes up and down. I automatically thought alternator but it tested good. However the battery was only holding a 25% charge. The battery is only 3 months old. The old battery was original and car would not start one morning when I got off work. Battery was determined bad. So the newer battery was tested and they said it needed to be charged but was ok. I took it off and they put it on a charger and tested again. Battery again tested good. I even had a different place check it. Battery was put back in the car and the problem has returned. I was then thinking the voltage regulator was bad, but I was told that the voltage regulator is built into the alternator on this car. When the lights dim and volts go crazy, the car sometimes almost bottoms out..lights almost completely dim out and car almost dies and then comes back. Any ideas would be greatly appreciated. Thanks.

Answer
Check all of your grounds. Make sure the terminals are clean and tight. I hate to throw parts at a vehicle but in this case if you check all of your grounds especially the one going from the battery to the engine block and they are good I would put an alternator in it. From what it sounds like something is loose. Check the power wire going from the battery to the alternator as well. Do a wiggle test. Have someone sit in the car and watch the volt meter and wiggle the ground from the battery to the block and the power wire from the battery to the alternator. If there is no change then I would change the alternator. Hope this helps.