f250 cobra 755 vibration/cavitation

[trickyrich]

Just picked up our cobra 755 second hand with a 2008 yam 250

Its our first big engine boat, previous being a suzuki 140, it seem to vibrate like a miss fire/ out of balance prop type vibration between 1200-3500 rpm, and it seems to make a cavitation type sound a lot.
i don't know whether both are related to a big engine just spinning the prop up, or whether the engine needs dropping and what is causing the cavitation if it is actually cavitation..

thanks
Rich