Sounds like the same issue I had with a Yama '96 C115. At low rpm the water pump worked fine. At higher rpm's it had reduced flow. In my case the water pump impellar had spun off it hub. At low rpm's the impellar caught and pushed water. At higher rpm's it spun free and had no bite to push any water. A water pump change did the trick for me. Hopefully this helps.
