After looking at the situation, if your grand prix really is devised with 10' of 8awg to the battery that could be the potential problem. That's tough to swallow though. I have OEM wires hooked up to my system and it was cranked out today without any issues. I'm running 4awg to the back powering two amps. Perhaps Matt has a point with the possibility that your alternator is suffering from reduced output. It does happen over time.
I guess an easy way to test that is by a big 3 upgrade. But remember, this isn't a fix all. It's a case by case basis. Also, be sure that you're power connections are all tightened down properly and that there is a secure ground point with plenty of surface area touching the frame/metal.
