Have you tried turning off the overdrive? Just push
the button on the gear shifter, the "O/D Off" light will
appear on the instrument cluster. Drive like you
normally would and see if the problem still occurs. If
the problem goes away, your overdrive servo is
going to most likely be the culprit. Overdrive is
basically your 4th gear, in your model, as in
mine(2006 Expedition), there is a retaining clip
inside the bottom of the transmission that has a
nasty habit of breaking... there is a great video
explaining exactly how to fix this problem with a list
of parts that will be needed... this is a great option if
you are mechanically inclined... The video is on
youtube and the channel is
(FORDTECHMAKULOCO)... Check out the channel,
this guy explains a lot of things that will help you
with your Ford... Good Luck!