It is probably your temperature sending unit. When they go bad they are likely to send out bad signals and if you don't get it fixed soon it could cause you other problems down the road. Remember that these cars are al computer controlled and when the computer reads that the car is overheating it makes adjustments to fuel and air ratio to compensate for the excessive heat that is really not there. Get it fixed. the sending units are not that expensive though I can not tell you exactly where it is located.