When i turn my volume up past about 25 my subs lose power, what could be the problem?

I have 2 12in kicker cvr dual 4ohm, hooked up to a alpine m6 amp putting out 724 watts rms. I have 8 gauge power and ground wiring. When i turn up the bass to my subs, they lose power, they dont go completely out, but they dont hit as hard, and then after about 10 or 15 seconds they go back to full blast, but i cant turn my bass up any higher on my head unit because then they dont stay hitting hard, i was just wondering because i had read a couple things, and it sounded like i needed to get bigger power and ground wiring, but i wanted to get some opinions first, thanks guys!