Does this happen if you go into forward without the 15 to 20 minute engine warm up?
Does it happen consistently or is it intermittent?
Do you have a folding prop or fixed pitch?
If you look at the prop shaft does it look like it is chattering or slipping in and out of gear, does it appear to be buckling/bending?
No to the first question,.....seems to happen consistantly now,......I have a fixed prop, ... and I have unfortunately not been able to look at the shaft yet when this happens.......it's way down under to get at and the vibration scares the hell out of me, as though it wants to throw the engine off it's mounts.