Hi Chester,
Have you confirmed that all of the main channels are operating at similar output? Try the Dolby Digital pink-noise setup signal that rotates from channel to channel and make sure they are at roughly the same output levels.
Perhaps one of the channels has become intermittent or you might have inadvertently changed one of the level settings for one or more channels. It's possible that one of your front channels may have cut out totally.
In a correctly operating solid-state amplifier, it would be most unusual for it to "lose power" over time. This can happen with tube gear as it ages, but not transistor equipment.
Regards,
