My First 285 has a Volvo 18hp, 2 cyl, Model 2002. There is a small vibration in the boat at cruising throttle (5 - 6 knots), and it comes and goes depending on the rocking of the boat in waves. I have checked propeller shaft for bend, done engine alignment, replaced the cutlas bearing, and assured that the motor mounts are secure. I am guessing that the sway of the motor is causing enough mis-alignment to cause the vibration, and to make it come and go. Should I replace the motor mounts? What about one of these flexible couplings between the engine and the drive shaft? Are they a good idea? Might fix the vibration?