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I have a 1997 Ford F150 and the I've changed the water pump, thermostat and got a rad system flush. When I start the truck, the heat gauge rising to normal then when I drive away the heat gauge goes to cold building a pressure in resevoir. Driving for about 4 km the heat gauge returns to normal. While driving the heat gauge and heat goes up and down.
sounds like there may be a loose/ worn wire near the switch. hopefully you have a service manual or are otherwise able to find the wiring id say check it out with an ohm meter.
it would help others who have had the same problem to know the engine and generaly more about the truck.
someone will come along with more info im sure, also check out the late 90s forum?