Could also be a stuffed battery??? It would be easier and cheaper to try another battery if you haven't already done so. Just a thought...
If you intend to change batteries, go for a heavy duty battery. I use the Optima sealed batteries and they are color rated and yellow is the way to go. That is what they use in taxis to cater for all the extra electricals they are running as well as being used in heavily fitted out cars running serious audio. They are not cheap but worth it in my opinion. Not much change from $500 though.