Prop Vib
Have you tried balancing it at a higher RPM , we had a scimitar on a Cessna 210 that was fine at 2500 ( crz ) to 2850 ( max t/o ) but below 2450 it was rough , we lost nothing by running at a higher rpm and the aircraft had factory engines go to life at this rpm
Good luck let us know what you find in the end.
Eng