2001 Yamaha 150 HPDI, won't exceed 3500rpm

I've had this motor for 10 years going on 11. Not one problem until this year. After putting boat in the water, it seemed sluggish, no power, slight vibrations at low speeds. My first guess was fouled plugs. I have not replaced them in about 3 yrs. On first run with new plugs the motor seems better, planed quick, RPM got up to 4200 w/o a problem. after trolling for about 10 mins it was back to rough running and would not go above 3500 rpm. 3 of the 6 old plugs seems to be a bit too clean, even though most years they all looked clean, but this tiome 3 were shinny. Now I'm think head gasket. I was told by mechanic that VST should be the cause and should probably be service, as a starting point. Any other suggestions?

Re: 2001 Yamaha 150 HPDI, won't exceed 3500rpm

Hi, Just wondering if you got to the bottom of this problem. I think I have a similar problem ....@ low revs a lot of shuddering and she wont run above 4000-ish RPM. Problem seems to have appeared this weekend after wake-boarding. 2 of my plugs seem to have some slight white corrosion on the outside near where the plug joins the engine and I felt that the leads were a bit loose......however i dont think this is my problem. Engine only serviceced in April and has been used for 10 hours.