I've always assumed this phenomenon to be one of those right/left brain thingys. Sometimes when you're playing you suddenly can't remember where you are (in the piece-though in my case...), but if you stop, clear your mind and sort of start over without thinking about it you can play through the piece without flaw. I think it's because many of us who have played for many years are so comfortable with the mechanics of our instrument that we don't take the time to really learn a piece. In our brains that means that we haven't "cross-referenced it in our right/left brain filing system. Thus, we can either "feel" a piece or we can recall a piece by analyzing structure, but we can't do BOTH. So we'er stuck there in a soup of embarrasment--unable to play something we could play with virtuosity the day before.

John