I will try to look at the problem in a slightly different direction. During performance, two processes take place: the physical and mental, which are starting to coordinate from the very first moment. At first, coordination with each note is created, and the general tempo of thinking in relation to the piece is super slow.
Gradually, from individual notes, thinking creates a sequence of patterns, first small ones - motives, then more and more large ones - phrases, sentences, periods. The tempo of notes is accelerated several times, but the thinking remains in the same mode of calm. It is clear that each pattern receives an initiating hook, then an arrow aimed by thinking flies through the air, falling into next hook. From hook to hook, from hook to hook. With the automation of performance, arrow flight distances increase many times. The normal situation is that the hooks points become stable, although an arrow sometimes changes direction a little while flying - because of the wind emotions . In my case, the stability of the hook point suddenly ceases, for no apparent reason; and only in this particular place.
Perhaps it is somehow connected: I sat down for the first time at the computer at 62, and for a long time I spat blood, until started typing in some way fluid, albeit slowly. However, periodically (every 15-20 minutes) the following occurs: I completely lose the sense of keyboard panorama , as if seeing it for the first time , and in these 11 years, nothing has changed. What a happiness that this isn't happening on the piano keyboard.
Not yet ...
I think I understand what you are describing. By "hook", are you referring to a moment where you are consciously aware of what's happening?
It's normal to expect these hooks to remain stable. This is how we make sense of the world. If we didn't expect stability, we would be forced to make conscious decisions every moment of every day.
The problem is that they will not remain stable. Yes, age can change them. Many other things can change them, as well. For example, stress, tiredness, anxiety. Also, any kind of learning. Increased awareness of the mind and body can produce insight that destabilizes the sense of security that we come to depend upon. Intermediate meditators often get very discouraged when they notice that the mind is far more chaotic than they thought it was. It feels like a regress, but it's actually progress. I have no idea what exactly is causing it for you, but it's important to keep in mind that these things DO change.
In my opinion, piano practice should take this into account. The brain will create hooks when it notices something it can really grab onto. And, those hooks will vanish for no apparent reason. The question is: what are you going to do about it? You don't know why the hook disappeared (and you probably don't really
know why it was created in the first place, even if it feels like you do). What often happens is that when a hook disappears, we panic, and in the process we stop paying attention to what's happening right in front of us. The irony is that this makes it impossible for us to notice what caused the issue in the first place.
It sounds like you want the hooks to be created in a predictable order. You want to start with the tiny details, and then work up to the larger patterns, step by step, in a smooth progression. That's great. But, have you tried the other direction as well? That is, starting from the big picture, then working down to the smaller details? You've tried a slow tempo, but have you tried a fast tempo? You've tried slow, calm thinking, but have you tried frantic thinking? I'm not saying any one of these is better than the other. But, each will give you a different perspective, and giver you brain more and more opportunities to make whatever hooks it can. You want to build redundancy into the system.