1996 F250 erratic temp gauge
new water pump,thermostat,thermo sensor,rad cap. Dash gauge cold to hot
back to cold constantly. New coolant filled. Bad water pump?
I suspect the thermostat even if it is new. Make sure there is no trapped air in the system.
Is it actually hot/cold? Full is correct an air pocket will give you crazy gauge readings but make sure its actually changing temp. Its an old truck the gauge or its wiring may also be a concern.