Mep 804a starts at 60hz then drops to 50hz

I purchased a unit with 20 hours on the clock last year after hurricane Michael. For the most part it ran fine for all of 320+ hours. A couple of times I noticed it would drop to 50hz for awhile but it only happened a day or two.
I start and run the unit once a month for a couple hours. Now recently a couple minuets after start up it will drop from 60 to 50hz and just stay there until I restart the machine.
Suggestions anyone?

I would start with the frequency select switch.

27. FREQUENCY SELECT switch
(MEP-804A/MEP-804B
only)(Located on controls
bracket assembly)
Allows selection of 50 Hz or 60 Hz.

SYMPTOM
17. Frequency meter (HERTZ) fluctuates.
TEST OR INSPECTION
Check back of frequency meter (HERTZ) for loose or broken wires.
CORRECTIVE ACTION
If wires are loose or broken, notify next higher maintenance level.

DO NOT; REPEAT; DO NOT; MOVE THE FREQUENCY SWITCH WHILE THE GEN SET IS RUNNING! Failure TO HEED THIS WARNING CAN CAUSE YOU TO DAMAGE YOUR GEN SET.