Possibly the thermostat is messed up. Does the motor actually overheat, or is it just the gauge? If the thermostat doesn't divert to the radiator once the motor warms up, it will continue to heat up and eventually overheat. If this is happening the gauge will rapidly rise to max.
Does the gauge instantly peg to max, or does it just rapidly rise there once the motor starts? If it just pegs there instantly, there could be a short somewhere in the gague's circuits, or the gauge could just be on the fritz. Head to a junkyard and grab a gauge panel off an old truck, and swipe it's temp gauge. They're all the same, just plug in and go.