loss of battery power

[suicide commando]

i have a 93 j30 with an aftermarket stereo/dvd player. with that i have an amp and a 12" sub attached to the sub output of the stereo. when i turn up my stereo while driving it works fine. but when im stopped at a sign or a light the sub cuts out. even more often when i have head lights or fog lights on. it also does the same when im parked and im rolling up the windows. i know its not the amp shutting off due to power issues because when an amp does that the light on the amp shuts off and in my case is not. i can tell its something to do with the overall power but im not sure if its the battery or if it could be something to do with the alternator not producing enough power. any ideas?

[fourdrinie]

http://www.aa1car.com/library/battery_runs_down.htm

[suicide commando]

So the battery checks out fine at 12.9 and so does the charging system at 14.33.I just installed a new stereo and it still does it. Maybe something wrong with my amp? I don't know. It's weird.

[suicide commando]

Also the amp is getting 13.7v while the car is running. I'm starting to assume its a bad amp