This is a stress thing, I assume you are nervous about it. Try to relax when you play it, the more worked up you get the more mistakes you will make. Can you narrow your practicing. ie. work on the first 10 measures, this practice session? Or maybe concentrate on one aspect at a time (phrasing, dynamics). Have you tryed it really slow, very carefully watching your fingers making sure you are not learning some notes wrong? With the metronome? Try working your way up from a slow tempo. You probably learned it that way from the beginning, but it will help to try that again. I know it's frustrating to go backwards, but it really could help a lot.
I think some of these ideas will help...when is the exam?