I have a 2000 21V with a 350 Mag MPI engine (940 hours). A few years ago the impeller reversed when I shut it down. It wouldn't start up again because the belt couldn't turn. Lots of high pitched screeching. I removed the impeller and reinstalled it the correct direction and it started fine. This happened a couple of times last season again, so I took it to a mechanic. He noticed some major problems...one cylinder missfiring, cracked heads. Over the winter, he did a head job. I've used it twice this spring, ran great. Third time running it, the same problem with the impeller happened. It starts, but the belt isn't turning, so it screeches. People often say that engine will just reverse the impeller if it's backwards, but that's not happening. Any idea what could be causing the impeller to turn backwards when the engine shuts down?