I have one crankcase and crank assembly that seemed to do the same thing, especially whenI  used an electric starter, but at other times also. I used some rough emry on both the crank, and inside the bearings, and then on the housing for the bearing, then used indutrial strength sleeve seal (similiar to thread locker) and to date, the bearings and crank have stayed in place.  Worth a try. The sealer is available at most Auto parts stores.  Flathead