2011 sea Ray 240 Sundeck 300hp Mercruiser 350Mag

[Nozideas]

Our 240 Sundeck operates as usual for the first 35-40 minutes then starts alarming (however no read out)and the Tacho needle starts going up & down as though its missing for a split second:huh:, the longer you keep going the more this happens. We have sent a diagnostic report to Mercury and have not heard back as yet.
Has anyone had this problem or know how to fix ?
much appreciate any comments
Thankyou.
Cheers Noz.