50hp Mercury 1979 Thunderbolt 2 stroke steaming when up on plane

[ckangaroo70]

I have 1979 Mercury Thunderbolt 50hp that runs great. Starts easy and pumps water good at idle up to about mid RPM range. I have installed a new water pump and all new housing twice because I thought perhaps something might have been defective in the first one. After installing the second new pump I am now convinced the pump is operating fine. However...something is causing it to lose its stream and it will start puffing steam out the water line when the boat is at WOT and on plane. I am pretty certain I got the impeller key installed ok which could cause this issue if improperly installed and I have ran string trimmer line up the line to see if there is some ort of blockage from an old impeller chunk or something, but it appears to be open. Just running out of ideas an thought maybe some of you guys a heck of a lot smarter then me regarding these things might have another thing I could check? I appreciate it.