Several mistakes here..

If the fluid had never been changed in 150K miles - your mechanic did
you a disservice. Changing the fluid dislodged some crud (tech-term)
that then circulated around the transmission, stopping in rather bad
places and causing your problems.  To fix it - at a minimum the valve
body and solenoids will need disassembly and cleaning. At worst case -
the entire transmission will need disassembly, cleaning and rebuilding.

OK - second mistake - if this is a US/Canada car - the transmission is
the GM/Strausburg one BMW installed for the colonies. It is very similar
to a number of other GM transmissions used in other cars of the era, and
 shouldn't be a mystery to a rebuilder. This transmission is designed
to use Dextron-II (which has been updated to Dextron-III), which costs
about $2/quart. It will run just fine with this fluid - standard AT
fluid. Later models did use some mystery fluids - but not in '92.

Suggestion - have the fluid changed and the filter cleaned several times
in a row. This MIGHT flush out the bits of crud that are causing the
problems you're having. If that doesn't work - visit find a reputable US
transmission shop - some of them have a machine that power flushes the
entire transmission.. this might dislodge bits that are stuck in bad places.

HTH

Go to someone who knows what he's talking about.