After I learn a piece, I can keep it at a performance level for a while, and then it starts to significantly get worse. No matter what I do, I can never get it back to performance level again. Sometimes, it gets worse before I can even finish learning the piece.
Is there anything I can do to prevent this?
How much time do you spend on foundational studies, like etudes and scales?
In am hour to two-hour long practice session, I spend about fifteen to thirty minutes on scales and études.
My guess is that you're going through the motions rather than working on the piece in detail. It's a very common tendency when a piece gets familiar. I'm definitely guilty of that myself sometimes.
While I would not put in quite so stark words I recognize the feeling. I believe there are several effects at work here:
At first I thought, "no way, what you're describing doesn't make sense" and then I realize that I'm going through something quite similar.
Albrecht Zumbrunn - I agree with you, but my decline usually happens when I'm learning the piece or right after I finish learning the piece. By "right after" I mean maybe one or two days after I finish the piece. This is not enough time to have significantly developed as a musician or to have lost detailed memories of the piece.
George Wells - Of course. If I pick up a piece that I have not played in a long time, I will have to relearn it; I know that.
What can happen; Your hearing, perception, standards, can improve faster than your technique, so that you think you are getting worse while you are actually improving. A piece can get stale, worn out, from too much mediocre repetition; just put it back on the shelf for a few months while the music percolates in the back of the brain.
Joel's on the money, I think.
Joel is correct, of course. It's the catch 22 of (more improvement) = (noticing more problems) = (us perceiving ourselves as worse before).