I've got an F140 4x4 (Off road package, extended cab) with 105K miles.
I live in Minnesota where it's been quite cold of late.  As expected,
the tranny takes a while longer to warm up and thus to shift into
overdrive.

Recently, this effect has been more pronounced than I'm used to.

I just had a tranny flush done and after getting it back I've noticed
that not only does the tranny take a long while to decide to shift
into overdrive, but it also seems to want to pull back out of
overdrive when I take my foot off the gas while still rolling along at
60+ MPH on the highway.

A ford dealer head mechanic told me that most likely this is the
beginning of a serious failure of the transmission and without
dropping it and inspecting it, there is no way to tell what might be
failing.

At this point my strategy is to let it ride and see if this behavior
goes away when it warms up, driving it carefully until that point.

I'm wondering if anyone might be able to give any feedback regarding
a) the symptom I'm describing above - i.e. that of dropping out of
overdrive at high speeds and b) the ramifications of my strategy of
waiting for it   to warm up and see if the symptom goes away.

Thanks in advance for any feedback!

-Craig