no power

I own a diesel 2500 2001 after starting up and going down the road the pedal goes to the floor and tach creeps up slowly whil the truck chugs along for 2-4 minutes than it drives normal until I get to an incline than it may or may not do it again after it runs for a few minutes no more problems I havent ever run out of fuel .help me out?:banghead:

Do you let it warm up?

yes,on friday I had no problems great power and then sat back to the problem again.