I have a 1990 454 Mercrusier. I start losing RPM's, power and speed after running hard for about 5 to 8 minutes. Once I reduce my speed to an idle for about 2 minutes and can get back up to max speed until this cycle (lose of power) starts over again . Any ideas why this is happening? Could it be that I have a 165F thermostat and not the 143F thermostat

you might be lossing power cuz your running the engine out of petrol

Sounds like fuel related. Have you changed the filter lately?