Thanks for the suggestions, Arjen, but the problem continues.

I did move the antenna out into the center of the room, and it didn't seem to help. I haven't done anything with the output levels, but I'm skeptical--I have blink emitters, and when the RF is working I see flashes on both the IRF-6 itself and on the emitters. Neither of these things flash when the RF fails.

(I'm looking forward to the much-complemented RTI technical support--so far they've ignored my emails.)