Volvo Penta 3.0GLP wont go over 3000 RPM or 10 mpg at full throttle, after about 5 minutes full throttle it starts to pick up speed and drive normal. I then pull back on throttle and RPMs seem fine. when I stop it does it again. I have checked throttle body, fuel filter was replaced, Gas additive and fuel system cleaner added to tank along with fresh gas. haven't checked spark plugs yet. I guess my question is.... what are the situations that can cause these symptoms?