Definitely something is wrong. Things I'd do above what the mechanics have done (oil, and fuel filters).
1. Check the fuel filter yourself. I'd say it's at best a 50/50 shot that your mechanic actually "Checked" what was in the fuel filter by taking the time and pouring the contents into a clear container.
2. Plugs are cheap, change them out. While you are doing this, inspect the spark plug wires for corrosion. Here is a link to help read the spark plugs
http://www.theultralightplace.com/sparkplugs.htm
3. Inspect/Change as needed the rotor and distributor cap.
4. You don't say where you boat, fresh or salt. But, after 3 years you are due to inspect the exhaust elbows. They are easy to remove and inspect. Drain the water from the exhaust by removing the lower water hose to the exhaust manifold. Loosen the clamps on the back of the exhaust elbow and slide off the exhaust pipe on the back of the elbow. Remove the bolts holding on the exhaust elbow to the manifold. Inspect the gasket surfaces and the inside of the elbows for bad spots and corrosion on the gasket surfaces. Replace the elbows by cleaning the gasket surfaces. Use Volvo gaskets WITH NO SEALANT! Use of sealant will definitely void any warranty you have. Torque the bolts to their proper force. Replace the exhaust host on the back of the elbow and the water hose on the exhaust manifold.
Small leaks in the exhaust system can produce a lack of power because the water slowly damages the engine and you loose compression. I know this as I experienced it and had to repower.
5. Have the compression checked to assure the engine is producing the power it's supposed to.
6. Make sure you have no restrictions in the fuel vent line. This could affect your power as well. Power up, engine is getting starved of fuel.
7. If you haven't cleaned your spark arrestor, do it. I've snuck mine into the dishwasher before (WARNING: caused headache from wife's hand hitting side of head at high speed!).
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Jvalich
http://www.badcock.com'04 FW 288 Vista "Mental Floss"