Dodge Repair: 2000 Dakota 4.7L Runs Hot after Hours, foot trailer, light duty truck

Question
QUESTION: I only use my truck to tow my trailer, 25ft. The temperature stays center of the normal gauge for about the first hour of continuous hwy driving then over the next 20 min or so starts to climb up. So far is never gone over the normal part of the gauge but gets within a needles width. I have installed a Trans cooler and new thermostat and it still continues. What would cause the truck to run perfectly fine for the first hour and then suddenly start to climb after about 70 miles or so.?

ANSWER: towing a 25 foot trailer would be my guess
drive it the same with out towing  i bet it would not go up that high then.

---------- FOLLOW-UP ----------

QUESTION: Yes when towing my trailer and yes it doesn't do it without the trailer. So far, I've only towed my trailer about 1-1/2hours straight, I'm worried that if I tow it farther, it will continue to go up and over the normal. Something seems to change about a hour into the drive, my truck gets up to the middle of the normal with in about 10-15 mins or so and stays there until the hour mark, then starts to climb and doesn't seem to stop until I slow down or stop. I keep my speed around the 55-60 mph.

Answer
u want a dakota to pull a 25 foot trailer  is your prob  the tranny is over heating and transferring the heat to the engine.If u keep it up prepare to rebuild or replace the tranny  
real towing needs to be done with a 1500 or a 2500  not a light duty truck.