I'm experiencing the same problem with my 04 f250 SD. I had the trans rebuilt after it was slipping. After that i was towing a 3k boat up a step hill. It was running smoothly,then all of a sudden my rpm was about 3500 and my speed was droping. It just had enough speed to bearly get pass the peak.Ford mention it might be the turbo, but i just had it replace a couple week before. If i wasn't towing anything, it ran great turbo and all. I'm having the machanic take to crack at it and i'll let you know.