Welcome to JA!
Is this a electric or mechanical gage?
any other lights or anything come on?
any recent work on the engine?
Thank you for the information!
I would go get a after market gage from Napa or someplace and plum this in place of your factory gage, to see if there is a gage problom or if there is
a problom with your pickup tube on the oil pump.
I think your having a gage problom.
If there was a sudden change in oil presser you would have a check engine light and a shut down warning.
I have seen temp gage problems on your truck.
and reading threw some repair history on the 98 to 2000 fl there is history of replacing oil gages too.
You can contact a FL dealer and have them run your vin for any TSB s or updates. they might be able to pull a repair history too.
Let me know if i can help more!