Volvo 5.7 Gi-B Throttle Issues

[CAMERON268Vista]

Hi Everyone, I have a few issues with my Volvo penta 5.7 Gi-B in a 2002 Four Winns 268 Vista. I think all these issues are somehow related.
1-when I’m up on plane cruising my rpm/speed slowly drops so I give a little more throttle and rpm & speed go back up, but after a few minutes I notice both dropped again. After a few times I Eventually I run of room on the throttle so I’m maxed out and only revving say 3400 and going about 25 mph. And it will continue going down gradually. If I shut the boat down then start it again I’ll be back to normal but will drift down again after some time.
2-if I’m cruising at my general cruising speed (35-3600 rpm / 27-29 mph) and have to suddenly throttle right down the boat engine will stall. It starts back up every time and runs normal. If I gradually throttle down from cruising there is no problem at al.
3-this only happened once at the beginning of the season but I think it may be related - I was docking so just in and out of gear and all off a sudden the engine started revving up but I was not giving it throttle. It revved up to about 3500rpm and back down about 3 times. Not pushing the boat so not in gear.
Any help is appreciated- thanks in advance

 

[alldodge]

Check fuel pressure, should be 50 to 60 psi. Need to leave the gauge on it when this starts happening to see if the pressure starts to drop.

Might be the tank vent, antisiphon valve or a fuel line restriction. If the fuel tank vent line is a little clogged then the pressure starts going down as the pump starts drawing a vacuum. The antisiphon valve if sticking or corroded with restrict supply

Need a scanner on the motor to see if the Idle control valve is around 40% under normal idle.

Has the fuel filter been changed?
Pour the contents into a clear container to see if there is anything but clean gas

 

[CAMERON268Vista]

Check fuel pressure, should be 50 to 60 psi. Need to leave the gauge on it when this starts happening to see if the pressure starts to drop.

Might be the tank vent, antisiphon valve or a fuel line restriction. If the fuel tank vent line is a little clogged then the pressure starts going down as the pump starts drawing a vacuum. The antisiphon valve if sticking or corroded with restrict supply

Need a scanner on the motor to see if the Idle control valve is around 40% under normal idle.

Has the fuel filter been changed?
Pour the contents into a clear container to see if there is anything but clean gas

Thanks, I'll try this!!