From only what youve told me it could be a million things.
First make sure your ground is AWSOME. Its vital, drill a couple holes in a steal plate n screw it in there if you have to n make sure its grounded to something that makes unpainted contact with the greater portion of the car.
At those wattages it shouldnt be too much for the cars battery unless your running it off of a lawnmower batt er something.. But if you find it doesnt happen with all your power accessories/heater off n does happen when you have alot of stuff on, the battery might be going bad or the alternator may not be able to keep up.
If it happens only or mostly when the car is moveing, its probably a bad connection somewhere.
If it happens mostly/only when the volume is high, it could be your ground or the amps could be getting too hot which can be caused by alot of things.