Power Problem

I just installed two 12 inch subs and an 800 watt amp in my '92 318 is. Whenever I turn my radio up too loud, it cuts the power to the amp the the head unit, then turns back on again, and will turn off again unless I turn the volume down. And I've already had my battery die twice from, I'm guessing, the amp taking too much power from the battery. I've had to pop sart my car both times. Any suggestions on how to fix this? The wiring was already done for the amp when I bought the car, and the head unit was already installed when I bought the car, so I don't know how its all hooked up. All I did was install the amp and speakers.