Hi Bob,
Don't have one, but all I have seen are smooth. a few ideas are,
On ground -First thoughts are: prop balance, bent engine crank, glue joint-wing to /fuselage at LE, soft motor mounts.
Flying - push rod resonance, loose controls, again, glue joint @ fuse/wing LE. soft motor mounts, bent shaft.
or -
does engine run smoothly on test stand?
Just a few "thoughts".
Roger