I'd try to divide some possibilities, hoping to conquer:

-- Determine if it's a loading or engine malfunction thing by very
carefully marking the governed throttle/injection pump actuator position
under 'normal' operation. If this suddenly moves under the fault
condition, the engine is getting more fuel then. If it doesn't then it's
not load or engine power related, and it's a looseness / shifting
mechanical thing. Maybe coupling, or worse, a loose flywheel.

-- IF it's an 'increased load' thing, two possibilities: (1) Sudden
added load as mentioned by others. Is There an ammeter on this rig??
(2) Sudden intermittent engine malfunction on one cylinder. Sticking
valve? Sticking Injector?

I'd bet on #2 as it would cause strong vibration due to uneven firing.

Keep eliminating 1/2 the possibilities and you'll find it....

--
Regards, Terry King ...On the Mediterranean in Carthage