I have an 8KW Westerbeke diesel generator (about 12 years old) which
has an odd vibration issue at times. This happens most often under
moderate to heavy load and lasts for 15 to 20 seconds at a time. The
vibration is so violent that the guages are impossible to read and it
shakes the entire boat. Vital signs like temperature and oil pressure
look normal, there is no exhaust smoke, and other than the vibration
episodes, it starts and runs very well. There is ample raw water flow
and the electrical specs for voltage and frequency check out OK.
I hate to replace it if there is a simple fix.

Anyone have an idea what the problem could be?