riser temp hitting 168 onstarboard side

ok are you ready i have twins port motor temp would run at 140 after a while it would drop to 100 or so thought it was the stat. next ride i overheated and blew a head gasket replaced gaskets ran motor it ran good but 1 riser would get to hot to touch.pulled boat replaced water pump and pickup tube same thing. this is where it gets crazy i ran port motor to starboard and starb. water to port motor to eliminate drive problem the motors ran the same.starb.motor risers cool port at 92 and 140 degrees did the clear hose thing there seemes to be enough water flow.i swaped termostat housings the same thing happened. i slpit manifold and riser all is clear the are only 3 years old.then i took the manifold hose from the left side of the thermo housing and put it to the right side manifold and put the right side to the left and the problem switched this is making me nuts any clues thanks carmine