I've been having the same issue with my 2004 GT1. Pulled the alternator and had it tested and it was fine, although I had the brushes replaced, due to whining. Apparently the 2004 has a special expensive alternator, don't know why but on all the gm-parts sites I've checked its more expensive compared to 2003 (same dimensions) but 215 amps vice 205, and 2005-2006 that I've been told won't work in an 04' don't know why. I then replaced the 5 year old AC Delco battery-went with a diehard gold. Same problem, took it to the local dealer and its not throwing a hard code, problem is intermittent.
I've checked the cables and replaced the belt when I did the alternator. The belt tensioner seems fine, but didn't request the dealer to check it.
At a loss, eventually I'll be on the side of the road and then I'll know what's what
