might be over heating. try putting something that would cool it all down, such as a fan in front of ur amp...try it a few times too tho with different set ups, like one time have it with a fan and see how long until it mutes, then try it without the fan till it mutes, then try it with heater till it mutes
not sure if its the problem but sounds like it, because subs, speakers and other things tend to turn off do to overheating..