I think it's more the cruise than the tranny.. My corolla does that
when going up a fairly steep grade with the cruise on.
It will downshift twice sometimes, which seems to be overkill
to me, but thats what it does.. Now, if I'm not in cruise, it rarely
does that, or if it does, it will usually just shift down one gear.
What I do with the cruise on is to manually give it the gas slightly
ahead of time, and that way it won't usually do it.. Like I say,
it seems to be a signal from the cruise saying, "hey, punch the gas,
I'm slowing down", and don't waste time about it!!  When that
happens going up a grade, I can turn the cruise off, and it will
usually instantly shift back to higher gears.
That heavy punch in throttle could be part of what makes the
tranny think it should downshift.
If the cruise is set at say 70, and you hit a steep grade, if you go
slightly faster than 70, it won't happen. It'll cruise right up the
grade in O/D, unless it really senses it needs to downshift
to keep from lugging.
So like I say, it seems to be more induced from the cruise,
than the tranny itself wanting to do it. It will rarely do that
with the cruise off, and if it does, it's usually just one gear
down, instead of two. Course, my corolla is a 4 speed A/T,
not a five..
Anyway, it bugs the heck out of me when it does that, but
I've found ways to get around it, so it doesn't do it much.
IE: going up a grade, manually gas it, so the cruise doesn't
end up the signal to do it.
MK