My 2001 Dodge 2500 24v Diesel runs fine around town with out of a load and when I get it out on the freeway around 60 mph after 20 minutes it has no power to accelerate then I back out and accelerate some time its fine . Changed fuel filter 62,000 miles Use injector cleaner once a month. Any ideas ?

 

is it smoking when you attempt to accelerate?

have you checked the codes? key on key off key on key off key on and look in the odometer window? wait for it to cycle twice.