Marine: Boating, Sailing, PWC, GPS and Navigation - Mercruiser 140hp loses power at high speeds

The engine starts and idles good. When I take off and try to drive at high speeds, the engine loses power. When I pull back to nuetral, the engine will sometimes die, but not always. I can then take off again, but it will lose power again. I replaced the fuel filter on the fuel pump, no help. I then rebuilt the carburetor. When I first tried it I thought that it was fixed, however, after a few minutes it started losing power again. The only thing I didn't replace on the carb rebuild was the high speed plunger, I didn't know how to get it out. Is this my problem? The carb looked pretty crusty when I rebuilt it. Thought I would ask some experts before I go back into the carb. Any suggestions would be greatly appreciated.