had the same exact problem with my GTP. Took it back to the dealership i bought it from and it was a bad alternator. since they replaced it i havent had the problem since.
2006 G6 GTP Coupe
6 speed manual, white
Black eyelids, 10000k HID kit, aftermarket 18" rims, tinted tail lights, Cold Air Intake, custom straight piped exhaust from the resonator back, partial debadge, black arrowhead overlays, 20% front window tints, 5% rear window tints and tint bar, 12" Pioneer sub in sealed box, 1000 watt amp