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Hi and thank you for your question.
Cavatation is a result of excessive impeller to duct clearance. It is exactly what I was thinking was happening as you described it, then you confirmed it. Providing you do not have weeds, fishing line or rope wrapped around the prop shaft, the 2 most common possibilities to create this problem is either a worn or damaged impeller or a worn out impeller duct. Yamaha calls it a housing, impeller. Since the housing has a stainless steel liner, the aluminum impeller is the higher probability component to be causing the problem. Also understand sucking up a rock or shell through the pump assembly can bend a prop blade and cause the same symptoms, and the resolution is usually the same answer, replace the impeller. I am attaching a link to a parts diagram so you can see what I am talking about.
If you have any more questions, feel free to ask.