Basicaly, when I prepare a new piece of J.S.Bach. I do love Bach music but, when I prepare a new prelude and fugue, it happens to me that I make mistakes, one time at a certain bar, the other time at another bar... Even when I know all the piece and can play it well, mistakes arrive when and where I dont expect... and this makes me mad. Because it isnt because some technical dificulty. Just now I play well, one minute after I pay wrong and dont know why...
And it is not a problem of memory, because I can write the sheet music from memory... Why? Why? why? I dont know...