I have a 04 dodge 3500 with 5.9 diesel with a automatic trans. Noticed today as I was traveling through some hilly roads pulling along about 12,000 pounds behind me the temp gauge rose quickly after the trans dropped out of overdrive/lock up.. It went to the top end of the scale but then as I crested the hills and the trans shifted the engine temp gauge dropped back down to normal within a mile. Never did this before so I checked the radiator , oil and trans fluid. All were they should be. Now what could cause this to happen.