I have a 2008 F150 5.4L that I purchased new. The transmission started showing signs of trouble around 10K. It was rebuilt under warrenty by a Ford dealer at 31K miles. The truck now has just over 35K miles and the transmission is starting to slip again - specifically when going down a decline at around 25-30 miles an hour. Has anyone heard of or had experience with this problem. Any insight would be helpful.

 

Try, http://en.wikipedia.org/wiki/Torque_converter and see if it describes your trouble.
My 1986 F250 4x4, provided a 10 year nightmare of trying to have one repaired correctly. To this day, I believe the problem was the torque converter.

 

my 06 bangs in and out of 3rd and 4th hard under load