Ksig
Have you checked your prop shaft for even the slightest wobble? A good spin of the prop in neutral, while keeping an eye on the shaft center point, may indicate a wobble. Even a slight one can indicate some damage to the lower unit.
Some years ago I was pre-fishing for a tournament trail on a lake I was very familiar with. While side scanning a fishing spot, the engine died. Sounded like it hit a stump but I knew there were no stumps near where I was. Pulled engine back to neutral and restarted. Shifted into forward gear and engine died again. Raised engine and saw a two foot long 2×4 floating up next to engine. Lowered engine and restarted with no more problems or so I thought. I ran that engine the entire year under tournament load as well as all the pre-fishing.
Took the engine to my dealer at Rockwall Marine for the annual tune-up. Tommy Yetts called me and said it looks like the prop shaft is bent slightly. I told him to replace the shaft. He called me the next day and said the entire lower unit including the casing was trashed and asked what I might have hit. I explained the 2×4 ordeal to him and he was surprised the unit held together that long.
Good Luck.
Eugene Parker