Mine only vibrates significantly under high load forward. Full reverse is smooth as butter. This seems to me to indicates two things; one, prop probably isn't out of balance, and two, it's only higher load in forward due to prop more efficient in forward. I've considered changing the motor mounts, though tired out, none seem to be broken. What makes me hesitate is that I've read threads where most said changing motor mounts didn't really improve things all that much. Man I hate to spend between $300 and $600 dollars with no improvement. In replies above, one 3GM owner says his does vibrate, and the other says his is smooth under high rpm. Obviously I don't know what's wrong, if anything, and I really appreciate all the suggestions and having taken the time to reply.