Excalibur29

I have a problem with my 115HP motor over heating.

I have done the following to try to rectify the problem.
Replaced water pump. thermostats, removed heads to check the rubber plug water flow control positions & condition (all OK) and replaced the head gaskets. The original gaskets were OK and there was no evidence of leaks.
Checked cylinder pressures all 120 psi when hot. Also checked timing (OK)

Engine runs perfectly until the thermostats open at 143 degrees F it then slowly continues to rise to 195 degrees F and the alarm sounds and at the same time the water pump starts to lose pressure and will stop pumping completely if I continue to run the motor.

I assume from this that exhaust is entering the water passage and causing back pressure to the pump stopping the water entering.
I would appreciate to hear from anyone who was had this problem and if it was as I assume a leak into the water passage.