2003 Mercruiser Horizon 300

I have built a boat with a Mercruiser inboard installed. It was installed in 2003, but the boat was not launched until 2009. It has 10 hours on it. The first year it ran fine. Since then, when advancing the throttle past 2800 RPM, the sensor throws an error message to the engine computer of low cooling water flow. This causes the engine to throttle back and run at about 1000 RPM with a 2 beep warning signal every minute. The sensor has been replaced 3 times and I have replumbed the cooling water line so the filter is about the water line. The water pump impeller has been replaced. When running the water temp remains at 170 degrees. I and maintenance have been working on this during 2010 and this year to no avail. Does anyone have any suggestions as to what to try next?
repilot

can you give me your engine#

steve

Does it have the original impeller in the outdrive? Yes? Replace it.

What kind of filter are you talking about? Proper marine strainer? Is it rated for the right volume this engine needs? Has the strainer always been on the boat (make sure it is not plumbed backwards, common error). Also check to make sure the oil cooler is clear, that may be where your pressure sensor is located.

This is a true inboard. No outdrive.

This is a marine filter. I'll have to check and make sure it has the correct volume. The strainer was installed when I installed the motor. It is plumbed correctly. The sensor is located on the transmission water cooler.
thanks,