The last two replies are definately something to trouble shoot. I'm leaning more towards a gounding problem. Think of it this way. If you have a poor ground, the battery is going to have to push (amperage) that much harder to get the power to the amp. Having a poor ground is like you're trying to cram something into a small hole. Improve the ground by either scraping the paint away or the rust that has accumulated where you've scraped the paint away in the past will make the hole bigger allowing much more efficient electrical flow. This will let your amplifier be more efficient and produce cleaner power with less heat. That way it won't be cutting off as much.

Just my 2 cents.