Onkyo 705 receiver shuts off at higher volumes. I bought a brand new Onkyo 705 with a KEF 3005

Question

I bought a brand new Onkyo 705 with a KEF 3005 speaker system. At medium or slightly higher volume, the receiver shuts down into a standby mode after 10 or 15 minutes as if it were overloaded or overheated. I put a large fan directly in front of it, and I still have the same problem. The speakers are hooked up correctly, except perhaps the speaker polarity, which I can't tell because the Monster cable has both wires colored the same. I could sort them out with an ohm meter if that is the source of the problem. Any suggestions would be appreciated.