i had the same problem when i replaced my water pump. what i am in the process of doing is putting a mechanical gauge in there. i also got one from the Oreillys down the street. they could be bad sensors.
early 99 F250
its stock, at least thats what i tell people
i traced mine to the connection on the block. i sprayed it out with electronics cleaner and it seems to keep it mostly working. i'll decided whether to keep the band aid or find a real solution. money to be spent elsewhere....