If in doubt...
Cut two circles of sandpaper the size of the hub of the prop...glue the sides with out the grit on it together....drill a hole the same size as the crank shaft, install either between the hub and the prop or between the spinner and the prop....and give the prop nut one grunt and two grones of torque, (Just joking their)
Tighten well.....I've seen this work on the nastiest of motors that was throwing props....using 25% nitro...
Most of the problem is running the motor too lean and it's kicking back and loosing the prop nut....
Just two cents worth....