One of two things appears to be happening:
1) A protection circuit is tripping
2) The AVR is faulty

Chances are it's #1. Is the AVR hot when it shuts down? If so, it's likely a protection circuit. Make sure it's located where it can get a fair amount of air, not in a closed cabinet. I don't recall the spec's of the speakers, but it they are 4 ohm nominal, the AVR simply may not be able to push them at high output. It happens.