I own a 96, Taurus GE with 51K miles on it and is in great condition. Recently, I realized that after driving the car for 5-10 minutes, the temperature gauge rises slightly above the 1/2 way point and them goes down to the middle point or below that. This happens especially when I slow down or stop,and the gaugestays at the middle level at normal speeds. The coolant level is at full.The guage does not go all the way up to red leveland I don't believe the car is overheating. Is this a sign of a bad thermostart? Don't know much about cars. Please advise. Thanks