Its a 1994 22' Larson with a 5.0L V8 mercruiser. Changed main fuel filter, screen in fuel tank, checked anti-syphon valve on tank, and changed the filter in the line just before the carb. The engine idles fine, and revs normally when not in gear. Once in gear, still idles fine, but once it reaches between 1500 and 2000 RPM's the engine either sounds like it's about to stall, or it continues to surge up and down between about 400 and 500 RPMs. Once past about 2500, it smooths out and runs beautifully. But about an hour into running, the RPM's slowly drop off and even at full throttle, is bellow 3000 RPMs. If I slow it down, it's almost impossible to get it back up to any speed and easily stalls. Had to limp back to the ramp 2 times now. I am thinking it has to do with the carb. or possibly the timing. Any ideas?