warf, the connection integrity at the receiver may be an issue that isn't readily evident. I avoided 12 ga (used 14 ga instead) because of the size of the wire being stuffed into the receiver. I've also read that 12 ga, being relatively inflexible, can put a bit of a strain on the connections themselves.
In any event, I remember reading something (can't remember where) about someone having an issue similar to yours. He tried everything, but still shut down, even after checking the connections at the receiver. A suggestion was made to trim a few srtands off the 12 ga end connected to the receiver. There is no loss of "stuff" getting to the receiver. He tried it and the problem stopped. May be worth while to try it out.
Another thing you might try to isolate the problem is to disconnect one speaker at a time to see if you can localize the issue to a single connection - then you can try and determine if it is the connection, the speaker or the receiver circuit for that one loop.
