this may have been answsered before. but anywau here goes....

i have a 2002 dodge ram 1500 2wd with the 4.7l engine. it has 160000 miles. the problem i am having is whenerever i am towing anything if the outside temp is above 70 degrees the truck wants to stop pulling. rpms are high. if i let the truck cool off it starts out fine until it heats up again. if i am not towing anything it pulls fine. never a problem. i have just changed fluid and both filters. still have same problem. anyone have any suggestings?

thank you