Hi capt RKO,
Check that PROP ! if it's WOOD and it has been on the engine for a while, the wood has been crushed just a little each time you tighten the prop nut. Problem is, after sitting for some time the hub is crushed and the prop is actually loose. It can shear with a backfire or throw a blade at high RPMs. If the Prop is Old, Replace it.
Regards and Good Luck,
Ken AMA 1528