don't no my problem do you know

i have outback 99 with a indmar 310hp carb, so here my problem the boat go well at speed 0 to around 30 but when i past 30 and not touch the gas the engine begin to loss speed the rpm go down and take back is speed again lost is speed and again take back is speed
need help
thanks

Hate to ask the obvious, but is your perfect pass on and set to 30 mph?