my water temp gauge keeps going down and then going back up? It only happens on the highway when the rpms are constant, and it does not go down that much only a couple of lines on the gauge. There is a very small coolant leak on the underside of the water pump, only about a quarter size spot on the ground when it sits for 8 hours when im at work. Could this be the temp sender(the truck has about 120000 miles) or something to do with the leak? any ideas?