[sjcrum]

I had the same issue with my 2000 Tiara with Cummins 6BTA's. I replaced the master boards, all sensors, and everything on the tachs including rebuilding the tachs, same issue ( tachs jumps and false alarms). I finally inspected the wire connections on the coolant temperature sensors, and one had bare wires. I removed the spade connector, reconnected, and thus far, (knock on wood) the issue seems to have gone away. I did take it out in fall before pull out, got it up to temperature and in decent size waves, no tach jumping and no alarms.
Something to look at, I think the reason it mainly became erratic in bigger waves was the motion caused the bare wires to touch.