Welcome to the world of 'harmonic vibrations and frequency resonance'! Remember playing with a jump rope when you were a kid? Each kid on an end could shake the rope end up and down to either make the 'waves' in the rope get larger or cancel out midpoint. This is a simple excercise to demonstrate what is happening with the tail of your aircraft at certain engine rpm's. Using softmounts just changes the resonant frequency needed to vibrate the tail. Be sure and balance your propellor/ spinner combination 'properly'. There are threads elsewhere to demonstrate this. Make sure the tail structures are adequately glued/ braced. Too bad there aren't any inline 6- cylinder or 3- rotor wankel type model airplane engines!