Trouble description: Running DG frequency drops without warning: No 1 DG running on load, suddenly frequency

Trouble description: Running DG frequency drops without warning:

No 1 DG running on load, suddenly frequency drops and blackout happens.

Suspected governor hence I went ahead and swapped it with an adjacent DG still the same problem persists.

The DG runs solo at near full load upto 1000KW (max 1300KW) and responds well in general. But out of nowhere frequency drops and trips.

Measures taken: Mechanical side everything checked and found in order. During testing I put a large load quickly but DG found to respond well. Load sharing in parallel is also good.

Parameters (Normal/Present): Alarm: Busbar low frequency (causes blackout)

Present Status: Troubleshooting in progress.. only running this DG during the day.