Boy, really sounds like the receiver, but you know that.
Next thing I'd try is hooking it up from a different source. If that's a PIA, run a 50ft extension cord from a different room/breaker. You've done the cable swap for interconnects/speaker. I've seen wonky power affect just one channel. And on that subject, just for sh*ts and giggles, try reversing the FL/FR, just to verify there's nothing wrong with the actual speaker. It wouldn't be the first time.
Another thought is use some contact cleaner on the RCA outs. Back in the day I used to have a magic touch with my Yamaha gear and somehow was always killing switches/connectors. Good stuff to have on hand regardless.
Sometimes getting it to act up, and eliminating what doesn't (or as you've noticed, when it starts) can help narrow things down. Good luck!