2003 Ford Expedition temp gauge movement

Started having problems with the temp gauge moving from normal operating range to top of heat range after the suv had been operated and shut off for a few minutes. The gauge would go back down to normal after the suv was started. Recently the gauge spiked to the hot range, which started the engine to reduce power. I replaced the cyc head temp unit and the themostat. It seems to work ok now, except that the gauge still spikes up alittle bit over normal when the suv has been ran and parked. The gauge goes back to normal when the suv is started. Please let me know if you have experienced any problems like this one and what was the fix.