I doubt its the impedance setting, but if the 605 has one it would be in speaker set up>speaker settings.
Couple things that could be the culprit.
Are there any loose wires either on the receiver side or the speaker side? If any wires from the positive and negative terminals touch they will shut down the receiver.
Do you have adequate ventilation for the receiver? If there isn't any clearance above the receiver it could be overheating and shutting down.
I highly doubt this is the problem, but on some Onkyos the sleep button is right above the volume control, maybe you are hitting it by accident?
Let me know if any of those are the source of the problem and if not, we'll go from there.
Thanks!
Jason Hicks
Aperion Audio Guru