Jeff, According to some forums, etc. that I read possible causes are amp. shutting off, is over heating, speakers are the wrong impedance 4 Ohms instead of 8, too many speakers connected to a channel, damaged/pinched/frayed wire.
Disconnect all speaker wires from the amplifier, see if it still shutting off. If staying on, reconnect one speaker at a time, turn volume levels up and down as you add more speakers.
If amplifier turns itself off when speakers connected, check their impedance, check the wiring of the speaker that caused it to stop working.
If the amplifier is still turning off with no speakers or wires connected to the amp., it will have to be opened and checked for bad components. Good luck.
I hope this helped you out, if so let me know by pressing the helpful button.