I probably didn't make myself clear about what to check. It's OK if the rotor can turn back and forth a little- - - - -that's the centrifugal advance working, which is normal. If the shaft can shake side to side, more than just a little, it will allow the point gap to change. If that's happening, the only fix will be to change the shaft bushings, or swap for a better-condition distributor. Also check for the sloppy vacuium advance linkage connection, which would cause the same kind of problem.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!