I have a 1990 Force 120HP engine. When in water below 2000 RPM runs fine-above 2000 RPM in gear the motor bounces up and shakes. Out of water with hose hooked up no bounce. Transom at about 4 second interval will bounce again and again. Runs fine above 2000 RPM. Tried replacing Hub in propellar did not fix problem seemed to have gotten worse. Do you think this be an engine problem or lower unit problem? Can it be fixed without having to bring in to shop? Thanks for the help.