My thermostat gauge shows hot but the engine is not hot

I have a 2003 Cadillac Deville 117,000
miles. When on the highway the
thermostat goes up a couple of bar
and occasionally but then goes right
back down. Sometime it will go up all
the way and give all the alarms that the
car has overheated but then go right
back down to normal. I have had the
relay fans replaced. Pressure tested,
everything I know to do. Anybody
experienced this problem?