a couple things that may help.
i would check the ground wire on the amp itself, making sure to bolt it to the chasis of the car (sand the area to bare metal if needed)
also the RCA wiring may play a roll in it. I've had issues with just p[lain old shielded wiring, so i always get twisted pair rca's (www.darvex.com and www.knuconceptz.com are a great place to buy good wiring) also make sure to have the rcas and speaker wire running down one side and the power wire running down the opposite side...
hope this help