Engine engaging while in neutral

[WOLO]

Posting for a friend with a 2002 Formula 41 PC with twin 480 HP Volvo Penta Diesel engines. Both engines have approximately 600 hours and have downloaded a computer upgrade. The starboard engine has a history of engaging itself even though the electric throttles are in neutral. They engage and then disengage. A few weeks ago after starting and synchronizing both engines the starboard rpm's feel behind the port for about 10 minutes then a noticeable change happened where you could tell the starboard was running the same as port. Also, periodically during cruising, the starboard would slow and speed up. A marine mechanic has performed a "vodia" check and ran both engines for 1 hour. They are not able to duplicate the problem.

Any advice?

[Blue Stocking]

Don't know if these powerplants have electronic throttles synchronizers, but could this be a place for incorrect commands.