I strongly concur that the trans needs to be electronically diagnosed
BEFORE anything's done to it.

My mother's '95 LH New Yorker recently had a trans issue that caused it
to go into "Limp-In" Mode (it stayed in 2nd gear and would not upshift).
She called the dealer (who'd just installed an engine fan relay) who
told her the only real solution (they could guarantee to fix the
problem) was a reman transaxle.  They would not guarantee that a
fluid/filter change would fix it.

I arranged for her to take it to a reputable trans shop.  They did a
"Quick Learn" with the scan tool as it was sitting in the parking lot.
What this does, among other things, is see how long it takes the various
clutch packs and bands to apply.  The longer it takes for that to
happen, the more these friction materials have worn.  It's all
"pass/fail" sort of stuff.

In her case, with a little over 100K miles on it, they found nothing
wrong (wear-wise) and did a full filter change and trans fluid flush.
Due to the adaptive nature of that trans, plus the fact that it has
never had a fluid change, each Sunday I drove the car to lunch, it felt
a little better each time as the new fluid cleaned things up a little
bit, progressively, until it now operates better than new.

I don't know what fluid they used, but it obviously has a better
friction modifier than the orig fluid did.  The hunting (modulated
torque converter clutch lock-up) and lower speeds is also gone.

By observation, most of these Chrysler transaxles DO have some  quirks
about them, but as generally better than many people let on like they
aren't good or durable.  IF they were as bad as many people claimed,
then Thrifty and Dollar Rent-A-Car would have been out of business
renting Chrysler products.

The Chrysler transaxles have more sophisticated electronics than GM or
even Ford, plus the need for specific trans fluids.

The trans' electronics will adjust the pressures to compensate for slip.
IF you put the wrong fluid in them, then it'll try to make things work
as best it can, adjusting the pressures something like 400 times/second.
With Dexron (without the earlier-spec'd Chrysler additive), it'll keep
trying to keep it from slipping . . . . and hammer the friction material
from the plates with the pressure adjustments.  Chrysler-spec fluid is
now available EVERYWHERE, so there's no need to use incorrect fluid.

Some earlier UltraDrives were also prone to suddenly "not move".  In
this case, the adaptive nature of the electronics would adjust the
clutch apply pressures to compensate for clutch pack slip until there
was no more friction material on the plates.  So it would "be fine" when
it was parked and then would not move again when it was restarted.

So, getting the trans checked (electronically) is a key issue!  If the
Quick Learn shows no wear out of the ordinary range of things, then a
full fluid flush and filter change would be the next course of action.

As mentioned, the FAQ at allpar can be highly informative.

Enjoy!

C-BODY