3 hp Yamaha possible overheating issue?

Just got a used 3 hp Yamaha (I believe a 1989) model number 3sf. After running it in the water for more then a few minutes, the motors has less and less power and eventually dies. After letting it sit for a few minutes it will fire back up. We are thinking it's a cooling or overheating issue. I'm not sure how the cooling system works on this motor or even what to check for? Could my diagnosis be correct? How do I check the cooling system? Anyone have a service manual on this motor?

Thanks,

Doug D

So we took the thermostat housing apart and found no signs of water in the passages, which tells me the water pump. Does anyone have the instructions on how to swap the water pump on a 3 horse?

Thanks,

Doug D

Just found out it was a water pump impeller. All the blades were gone.

Doug D