The amp itself, as the source, could suffer an intermittent output problem. When near ground or high resistance is detected, they go into protection and snuff the output. I'd open the common (channel) connection points and use temporary by-pass jumpers for testing to see if you can restore certain speakers while others are disabled. May at least tell you where the problem is. Tweeters, mid-bass and shelf speakers have dedicated amp cross-over outputs from the amp. The amp therefore is multi-channel. I can't see why front door and the rear shelf combo fail as separate amps unless the entire channel source (preamp signal) is dying? Are there any left-side speakers that remain working?
Anything made can be made Better