overheating

Greetings,
The engine i'm working on is a 2005 200FETO two stroke Yamaha. It is rigged into a Seamaster 27 ft as part of a twin engine setup.
The problem is that the temp alarm goes off almost immediately after WOT or after a couple of minutes after running in the 3000-4000 range.
inspected the heads, replaced the thermostats, pressure valve and impeller. Problem persists. Don't know what to do next. Any Ideas?
Thanks and Regards,
Manuel

[boats.net]

Did you clean water jackets around cylinders when you had cylinder head off? Try a complete water pump, housing and reapir kit.

Getting better

Thank you for your response.
The water jackets were clean and the complete impeller was replaced including the housing. I did get an improvement on the condition by treating with SALT AWAY. After about 4 treatments the the alarm goes off only at WOT and after a couple of minutes. This week I gave it additional treatments and we're waiting for the seas to come down to go out and try it on an extended run.

regards,
Manuel