I had a similar problem with a center channel speaker many years ago. Turned out to be a bum tweeter that acted up after watching a movie for a while. Never noticed it with music since it wasn't playing or normal TV watching because the volume was always lower. Maybe next time it happens immediately run the calibration tones and see if a specific channel is causing the problem or if it's all channels. I would think the pink noise and a RS meter could tell you if a certain channel is having a problem. If it turns out to be a single channel at least you have a place to start.