Mine did the same thing. My cover plate (and back plate somewhat) were scored from wear. After turning over the cover plate (clean new surface), my pump worked much better, and no longer lost it's prime - like yours.
I believe that the water rushing past the hull, while sailing, causes (Venturi effect???) which helps to pull the water inside the pump and hoses (along with back pressure from exhaust possibly) You then get air and pump has to reprime.
Have a look at cover for wear, and if so, clean up other side and flip over. I used a very thin layer RTV (auto) silicon gasket instead of the paper gasket. You want it thin so that the impeller is as close as possible to the cover plate.