Stereo cutting out - is it my amp?

I have a dual battery set up with Perko switch and an ACR. After about 3 hours of listening with the engine off, the stereo starts cutting out.

Would this normally be due to one of my amps heating up? I don't think it's my deep cell battery b/c when I get home to put it on the charger it's still at 65%. If it is an amp, how do I figure out which one?

Here's my setup:

Head Unit: MB Quart Nautic There are actually two of these in the boat (1 in the glove compartment which is a CD changer, and the other is at the helm). http://www.mbquart.com/nautic_accessories.html

In-Boat Speakers: There are 6 Kicker in boat speakers. Not sure on the wattage, but they're the typical 5" or so circular guys.

Amp #1: Kicker DX200.4. I assume this runs the in boat speakers but haven't followed the wires just yet.
http://www.crutchfield.com/p_20611DX...200.4&skipvs=T

Amp #2: MTX TC4001 Thunder - Class D. http://outlet.crutchfield.com/S-JKfF...er-TC4001.html

Amp #3: Planet Audio AC 1600.4, 4-channel mosfet. http://www.vickersav.com/index.php?m...oducts_id=2792

Sub: Kicker L5. http://www.crutchfield.com/S-42RW0AL...-06S10L54.html

Tower Speakers: There are 4 Phoenix Gold tower speakers in Roswell cans.