8HP Cooling problem

[dingoboats]

I Have an 8 HP long Shaft 2 stroke.
Initial startup is fast and constant .
Water pressure at telltale is consistent then dwindles to a dribble, finally ceasing to exist but for steam/air escaping.
I have replaced impeller, checked housing and bottom plate.
Head cover plate has been removed and thermostat is clear.

Any clues on where to check next would be appreciated.

Thanks

PS - I have just read labougie 's post - alas it sounds like the same problem. So any further suggestions anyone ? Cheers .