strike one

Hello, I have just bought a 2008 f 250 with the 5.4 3 valve. I towed my boat (2500 pounds) 200 mile this past weekend. The truck is a crew cab 4x4 3.73 gears. It won't maintain 65-70mph, it upshifts and down shifts all the time, even in the tow / haul mode. Has anyone else experienced the same kind of problem? If so, did you find a solution to correct it? Thanks!