We have a 4.3L Mercruiser. We put a rebuilt prop on it and it was running fine until I got to a corner to turn and the motor reved up and would not go. I pulled back on the throttle to idle and started to go again and it slipped out again. If I slowly brought it up to speed, it worked fine until we got up to about 3000 rpms and it would slip out. Any suggestions of what might be wrong?