70 Force looses rpm after running awhile

have 70 force, new carb,new fuel line,new tank, good compression all, runs good around 3200 rpm for about 1/2 hr then drops to about 2000 rpm. motor doesnt shake and runs good at idle during that time so i dont think it drops a cylinder. fuel delivery is not a problem as the bulb is hard and except for rpm motor is smooth. is there a limiter or module heating up causing this?

Year and serial#?
Possible the fuel pump diaphram is weak/tore.
The tank vent open? Built-in tank?
Possible bugs have gotten in the line and made nests.

made 70hp between 91 and 95. id tag is missing from mtr. fuel pump rebuilt,new portable tank,new lines,new carb (not rebuilt),new coil packs,new plugs and wires. starts and idles good,throttles up strong and runs 3500rpm at ease. about 1/2hr in will drop to 2500rpm and run good at that. start back up after a couple of hrs and idles good,throttles up strong to 2500rpm and runs like that the rest of the day. the next time out it does the same all over again.
Doc

What kind of connectors do you have in the fuel line?
Did it do it before the new carb?Float level???
Comp good,what's the numbers?
The bulb shouldn't stay hard after starting.It shouldn't go flat but loose a bit of hardness.
Try posting on iboats they have a few other guys who can maybe help.J

it did it before new carb.. comp. was all around 105lbs, pulled and checked reeds-ok, connection are new mercury quick connects. worm gear to pump. i try to pump the bulb when it happens under power and you cant which i assume means carb is satisfied.