I recently bought a 1999 sportcraft 231 w/a, with a Volvo Penta 5.0 GI duoprop. I brought the boat to be inspected, and it rode fine. Shortly after, the boat shakes/shudders between 1000 and 2500 rpm. The boat will not go higher than 3000 rpm at WOT, and a knocking sound can be heard coming from...