I cant afford to take this in to a repair shop I justgot a job. The faster you drive the hoter it gets

My dodge dakota temp runs 210 arround 40mph and raises to260 at 70 mph and cools back down when returning back to 40 mph. If I turn the heater on full blastit cools down to 220 at 70 mph. Ive changed the therstat and the water pump has not helped