check the engine mounts. They could be shifting very slightly which would cause the mesh to change. your slipper cluch might also be to tight. you should loosen the slipper nut if it is. ATXPro4 also mentioned that the spur could be out of round which would also cause spur grinding if that is true try different brand spurs. a good base slipper setting is to turn it as tight as it goes then loosen the slipper nut 1-2 turns then fine tune from there