Suzuki 70 4-stroke will not power up

[fatlazyless]

Hi - I have a 2000 Suzuki 4-stroke outboard w/ about 450 hours that will not power up correctly. It will get up to 3000rpm in spite of the throttle being pushed all the way forward while underway. It should get up to about 5500rpm. Set in neutral, it is also limited to 3000rpm.

My question: is this likely to be an overheat protection feature and it's likely that the water pump impeller needs to be replaced or could it be something else?