Troubleshooting Mechanical Fuel Pump?

The problem started to appear about 4 weeks ago. At first I suspected bad gas or water in the gas, because it was after the one and only time this year that I purchased gas from a marina. Since then I changed the fuel filter and checked the inlet screen in the carb. I will take a look in the tank for an additional screen. I sort of assumed that good flow during the idle test would indicate that the fuel lines were clear.

The problem goes like this. If I have 3 or 4 adults in the boat, I can easily open it up and push the boat to 4400 RPMs. However, I have one friend that is a hard-core barefooter. He likes to stand up at 38mph, but once he is up he wants to speed up to 41mph for one-foot tricks. The boat used to be able to handle this. But now, when I open it up at 38mph (about 3900RPMs) the boat momentarily increases speed, hesitates, then settles in at about 39mph. If I hold the throttle full open, the engine will continue to hesitate every so often and the RPMs won't exceed 4000.

I didn't know if mechanical pumps could drop in pressure during high RPM's.

Thanks,
Mark

First what did you do to create this problem, or did it just appear?
Look elsewere, add octane booster run the boat in the water it will turn the plug tips red. If the plugs remain white or the same as they look now, you are lean. The only positive way to check pressure when running is to have a guage.
Under loads the carburetor demands different amounts of fuel, to elimate the demands they install a return fuel system like fuel injection.
However I'm sure you don't need this system, only for high performance.