They say its life time transmission fluid but its not! whats life time? when it fails due to poor depleted oil? I changed mine, filter & Castrol Dexron VI Synthetic and I also changed all the shift silinoids ( @ $130.00 for all 5 ) very easy to do whan you drop the pan.
Best thing I did, shifts up and down perfectly. will this fix your problem I do not know but it made a big difference in mine. I did it at 130,000 miles and it has 154,000 on it now and still runs great.
There is a electric silinoid that controls the down shift, they have very fine mesh screens which can impede fluid flow or just be worn and sticking when cold, I would start there if you can do this yourself its not a costly job.