I didn't read the other thread answers, just answering directly. This would happen to me after a few hours of playing, maybe in every fourth tournament or so, or sometimes when I would play friends. Sometimes they would even say "Nathan, are you alright?"
It had to do with my pre-shot routine missing something that should be a conscious part of my process, and instead I made subconscious out of naiveite. When this thing was calibrated subconsciously would play great, but when my mind got tired this piece would un-calibrate and I would fall off in a big way.
After years of studying my pre shot routine, I finally figured out that I wasn't making a deliberate effort to align the exact center of the cue ball to my shot line. I've played pool / billiards so much that I would just feel where the center was. But by not taking the extra second to just look at the top edge of the cue ball and verify that the peak of the curvature was on top of my predicted shot line, it allowed my "feel" of the center of the cue ball to drift very slowly as I became more mentally drained. This caused other problems like dialing back on max English due to a feeling of under-confidence, and things unraveled. I added this visual check on every shot, and also use my lower peripheral vision a bit more to watch my final follow through to make sure it isn't drifting towards center.
The issue with this is that it is so incredibly personal and internalized I don't see how any video or instructor could even pick it up. No training device that I could make could figure this out either. It is very mental and subtle, and just requires a lot of patience and observation to notice and fix details like this.
So I would say that all pros have gone though this process many times, and have PSR that is bulletproof, not only for every shot, but also checks micro details when their mind is tired and keeps them on track for the long haul.
