Yamaha 150HPDI 2003 Rev problem

Hello everybody, new to this forum and I would really appreciate if somebody can help, I have twin 150HPDI 2003 counter rotating in my boat, the starboard motor at times being running OK at, lets say 3500, suddenly goes down to 1800/1900 and kind of shakes a little and no matter if I give it gas it stays there, I have to turn it off and on again for it to regain revs and starts to work fine, it can do it several times a day or simply not do it for days at a time, what do you think is the problem? I have changed the fuel filters, the inner filter on the motor and the outer (water separator/filter) one on the boat, there is a little kind of check valve just before the inner filter on the motor and was changed as well, new spark plugs, premium gas, I have to mention that the port one works perfect same gas and manteinance, any ideas?
Thank you in advance.

[boats.net]

Need to run a fuel pressure test, may have trash in VST.

Vst?

Hello, thank you for your fast reply, I tried the FAQ and search option on forums (even google) for VST word reference and could not find a meaning that would apply to motors or boats, would you please inform me of its meaning?

Thank you in advance.

Vst

Got it, I changed the filter element a few motor hours ago a the problem persists, thank you in advance for more ideas.

[jb123]

you say it goes down to 18-1900 rpms....do you have digital gauges?? does your buzzer work?