Subwoofer cutting out at high volume?

I have a 600 watt 4 channel Sony Xplode Amp that I was using to power a 500 watt Kicker Comp, but it kept cutting out at high volumes. I thought it was because the sub was older (the amp is brand new, but the sub is a couple years old) but even after replacing the sub it has the same problem. I think it might be because of the gauge of speaker wire I'm using, but I need more suggestions. Before you ask, the amp does not blow, the light color does not change, and the cables from deck to amp are all secure. The only issue could be between the sub and amp (or battery I suppose but the battery is hardly a year old). My friend suggested I check my ground, but in that case how do I make sure it's securely grounded? Thanks