i bought a 98 lexus gs400 and its in great shape and runs great. but it has a custom sub system. he bought it a while ago and said the amp is something like 500 watts and it runs a decent 12 inch sub. heres the problem when i play songs with big bass at higher volumes it starts to sound like a skipping cd. and it does it when i use fm radio or cd. but if the song has very little bass i can max out the volume. so beatles plays loud rap music has to be quiet to not skip. (the car was sitting in his garage for a year if that matters) so is it the battery or alternator or what else possibly. thanks for any help.