Sluggish boat, then runs fine

[fourwinns2003]

We're having a similar issue-full throttle at start up only gets us about 10-15mph (tach not working but sounds like RPMs not getting as high as they should be) HOWEVER once the boat has run for 20 minutes or so and then sits for 20-30 minutes, it runs like a champ, 40-44mph, not struggling. no idle issues. volvo penta 5.0 GL engine, duo prop outdrive. new manifolds/risers and plugs. have drained gas (3+years old - boat is new to us) ran sea foam through it. changed outdrive oil. replaced fuel and oil filters, but haven't been able to get off the smaller in-line filter. Any ideas? thanks for the help!