Check the connections at the panel because moisture can get in. I have the opposite problem, my tach will max out at 4000rpm then fall back within 30 sec to cruising rpm and we have traced it to moisture condensation. My boat is in the water all year with a canvas pedestal cover which may be part of the problem. Rain is not a problem this year as we have been dry almost 2 months, and this is Seattle area, so we are looking at condensation. Good luck tracing the problem down.