Unit going into protection mode
You should first have or check the impedance and resistance of the speakers. then if all the speakers and wiring checks good, the only way to tell if this unit is good or the output section has no problems, is to hook up load resistors to where the speakers go, insert 400 to 1000 hz @.775MV into an input, hook up an osilliscope to the load resistors and slowly turn up the volume and see if the waveform clips before it shuts down. I doubt if you will have the equipment to preform these test but a good repair shop will be able to help you out. If it does not clip or go out then its all speaker wiring and/or the speakers.