I replied to another thread briefly discussing this, but I thought I'd share a few more details about another transmission issue that is now resolved.

My 1998 Vanden Plas has 116k miles on it. I uneventfully serviced the transmission (oil/filter) at around 100k miles. Daily driving and long road trips all went well in the car.

Then a couple years ago, it started acting oddly when turning corners (felt like the engine was cutting out and/or brakes were engaging). Progressively got worse, as it would shift out of gear at stoplights, Revving the engine a bit would make it engage, sometimes with a thud. I was also getting some intermittent surging when slowing to a near stop in traffic. That was extra concerning as I would have to catch it and apply more brakes before it moved toward other cars stopped at lights. It would sometimes stall when this happened too.
All these issues together made for a pretty unpleasant and somewhat undrivable car.

I initially thought I might have a problem with the throttle body or maybe the MAF. Both a little more expensive than I'd want to just swap them to troubleshoot. Also, the AJ26 has a more expensive MAF sensor than the later AJ27, so I wasn't too keen on swapping parts without further diagnostics.

With the neutral shifting and banging back into gear, I was also reasonably sure it was an impending transmission problem that might possibly be rectified with a new shift valve OR a replacement solenoid. I bought the Transgo 5hp24-PR valve; I prepped the new valve and put it in the freezer, as per the installation instructions.
That was 18 months ago. Other life events took priority, so the car sat there. I'd sometimes start it and let it warm up to operating temp, but I rarely moved it as I didn't want to deal with the stalling and shifting problems.

I got a bit more motivated recently and had some open garage space, so I finally dropped the transmission pan and lower solenoid and got the new Transgo valve installed. Took a few hours, but the results are remarkable.

I have driven it every day for the last week; the transformation is pretty amazing.
No more bang-shift neutral drops, no more surging, no more stalling. I knew I had some transmission problems, but didn't realize at the time that some of these other problems might have been related. Looking back, I guess it does make more sense as erratic up or down shifts can (apparently!) cause strange problems.

Anyway, thought I'd share a success story!

 

Hi pod184.
Glad everything is working out for you.
Did mine about 18 months ago now,very good upgrade.
Thanks for sharing.
Take Care.
Andy