You need to get away from relying on that muscle memory.
There seem to be five ways to approach memory.Some advocate using as many as possible at the same time, while others say to focus on what fits your personal style the best.But all that I've read agree that muscle memory is the least reliable of the five.
EDIT: Memory lapses tend to happen in "easy" passages that don't require too much attention. Make a TECHNICAL challenge out of it (voicing, getting a melody out, etc.) and your chances of successfully remembering that passage increase considerably!