What are you running your motor for 2 to 3 hours for? Easy fix.
Just kidding, happens I know.
You say you cleaned the exhaust manifold. Did that include the mixing elbow? If clogged, you would not get sufficient flow. I do not think it is possible to visually gauge the amount of water coming from the exhaust, only whether you have any.
You RW pump may not be doing enough work. A one year old impeller is ready for replacement in my book, even if it looks good. The interior of the pump can be worn and not pumping at max efficiency or, if it's belt driven, the tension may not be sufficient. Although, too much tension will wear the belt out. The suggestion to replace the cam, if serviceable, is possibly a good idea.
Again, just because you see lots of water in the exhaust, does not mean there is enough.