2000 Sierra bang shifts sometimes, what's broken?

I have a friend with a 2000 with a 5L 4speed auto. She has had this truck since new and it has around 100K on it. It usually runs fine but sometimes once it is warmed up it tends to bang shift through the upper gears. Let it cool down and it is fine and it may or may not come back, it is random. Any pointers on what sensor needs changing or what other remedy is in order?

My 2000 did the same thing check this out:

http://www.automotiveforums.com/vbulletin/showthread.php?t=952352&highlight=hard+shifting

P1870 DTC stored? then you have a worn out Valve Body.