Timing Belt Jumped

I have seen a timing belt jump time a couple of teeth in the winter because a little bit of slush/water/snow got under the timing belt cover and forze. In the case I saw, the cause was probably a missing splash shield that allowed more winter grunge than usual to hit the engine and work its way under the timing cover.

This may or may not have anything to do with your problem, but I just wanted to note that it certainly is possible.