How do i keep my starter from jumping I have had to drop a bolt from the top

What is your question?

Mounting threads have been broken. Bolt has been dropped from the top. When I tighten the nut on the inside mounting bolt and the out side up evenly the bottom of the starter does not stay flush with the bell housing. how do prevent that from happening? Any help would be appreciated What seems to make the problem better or worse? When flush starter works fine but keeps jumping out of place How long have you had this problem? A few days