I think all you are feeling is called grade logic. This tranny when slowing down or going down a hill will down shift and will try to hold the speed constant while going down a hill.
" I believe the Grade Logic is a means that the tranny senses you are going downhill and applies some engine braking as you would by downshifting a manual tranny. Many automatic transmissions would pretty much just freewheel going downhill and allow your vehicle to gain speed and to slow down that acceleration you would need to either downshift or use the brakes.
I have felt this engine/transmission braking a number of times on a downhill slope. "
http://townhall-talk.edmunds.com/dir.../.eea1204/3185
The only other thing I can suggest doing, is dropping the pan, changing the filter, top it off and pour in some tranny stop slip or some other tranny help in a bottle. Also it wouldn't hurt to clean out the range sensor, which sometimes causes the tranny to shift harshly (see the faq's for the how to on it).
http://isuzufaq.ibctech.ca/#faq59