HELP! Completely stumped
4JH3-THE; Kanzaki Gear box; 3000 hours on the engine about 12 years old; Installed in 21' Pompano - The boat wass in daily use and the engine was running fine until I pulled the engine to fix small oil leak found in a return hose to sump. While engine was out I replaced (as precaution) the raw water pump; damper plate on gear box(as precaution); and adjusted valves (were chattering a bit). Replaced broken engine mount and other three as well (as a precaution). Checked shaft (OK); prop rebalanced and blades realigned. Shaft and prop turn easily. Oil level in gearbox is ok. All seemed good to go. Engine ran great out of gear.

Problem: Back in the water if the engine is cold (15 - 20 degrees C), no issues occur and I can run up to full revs (3000- 4000 rpm) and back down to an idle. Once up above 70 degrees C on temp gauge however, when you gradually slow the boat to 1100 rpm, the engine instantaneously develops terrible vibration/shutter at that point and will it not stop unless you let the engine cool. Engine vibrates even at idle speed. It feels like it is running on 2 or 3 cylinders. This does not occur if engine is out of gear or in reverse. At just at or under 70 degrees C, id shuddering, I might be able to rev up and down and actually 'push' past the shudder... in that case it instantaneously stops shuddering and runs smooth again until I slow back to 1100 rpm. Once engine is at full operating temp however, there is no pushing past the shudder - it feels like it will know the engine off the mounts!I have seen other people report similar or identical issues, but have never seen a final reply to say how it was fixxd Any suggestions on how to work this problem would be greatly appreciated! Thanks.