Thanks guys.

Westerbeke would take issue with the "cheaper unit" (so would my
wallet), but I'm sure it's all relative.

I was surprised to not find an electronic voltage regulator.

I will check the frequency this weekend as a first step.

What I do know is that it used to do this right before I bought the
boat. A gen mechanic did something that fixed it, we completed the
sale, and now 4 months later it's doing it again. Now, if I can just
find that mechanic.