Raised volume cuts sound and gives me...

Question

I have hooked 4 speakers to my SA-DX950 technics and when I raise the volume up to a certain limit the sound cuts off.after 5 minutes of play. note that I do not raise it all the way up. I have 2 speakers to the main L/R and the other 2 to the lower front L/R. Could this be the problem?. The receiver is suppose to be 500wts