I don't know how to find out.

Do you have a portable player (like a CD player or a walkman) which has a line out jack, and do you have the proper adapter to connect it to the RCA plugs on your amp? When the problem happens, go to your trunk (or whereever the amplifier lies), unplug the RCA plugs from the amplifier and plug your portable into it instead. Does it play fine, or does it exhibit the same problems? If the latter, then the problem is the amp, its wiring, or perhaps a problem with the blue remote-turn-on wire where it connects to the empeg.

Also, definitely check to make sure you don't have this problem.