I know that this thread is kind of old but my truck does the same thing.I think that it is just because of where the sending unit is at. Mine is between the radiator and the thermostat. When it opens, the hot water from the engine hits the sending unit and it spikes. Once the cool water from the radiator is circulated though the engine the water temperature at the sending unit will be cooler and the gauge goes back down. I think when the sending unit is in the other end of the engine the gauge won't show the spikes like that.

Gene
1959 Apache