System shutting down due to clipping?

From time to time my HT system shuts off when playing at higher volumes. The Yamaha AV unit is running out of juice and I assume it is tripping off as a self preservation function. I am guessing that the rear surround speakers are the culprits since they are 4 ohm. I am not sure whatload the center channel is.
SO how do I fix this? I do not want to change the AV unit and am not real keen about changing the speakers either. Don't even think of suggesting that I turn the volume down.