Salutations:

Sorry to top post - but the details of your problem are important and
probably should be left with this lengthy reply.

Basically - an automatic transmission is a complicated high pressure pump
fed by the torque converter which powers a fluid/ hydraulic drive
solution.  Unlike a manual transmission - there is no direct metal to
metal connection between the engine and the differential until the torque
converter 'locks' and/or 'stalls' under very specific conditions and even
then a true direct engine/differential connect is debatable.

Normally - the valve bodies channel the pressurized oil at different rates
appropriate to the speed of the differential and engine RPM. Primary
pressure being developed in the torque converter is passed through a
liquid circuit to a various sets of clutch packs (if you've ever owned a
motorcycle - they look sort of like the clutch basket on a much smaller
scale but longer length - like a long stack of poker chips).

The pressure developed by the torque converter is appropriately metered
through the circuit(s) to the clutch packs using spring valves, vacuum and
in some cases electronics - against a known fluid viscosity and pressure
in the circuit(s) (different transmission fluid types) so that the
transmission can be pre-set to use different circuits as appropriate to
the load feedback being felt through the differential and the pressure
being applied by the advanced rotary pump that is the torque converter.

When an automatic transmission starts to miss or jolt it may be a single
fault in the circuit - or a combination of many smaller faults in the
circuit(s) - or some greater fault outside the circuit. Given the really
quite deep diagnoses time required to figure out which, or which
combination, things are generating the fault and the fact that the problem
may additionally complicated by the fact that deterioration in one part of
the circuit may contaminate another part of the circuit so that it's flow
characteristics change - most folks opt to simply replace the transmission
and/or sometimes the entire power train with a known complete good unit
when a deterioration type fault develops.

This is premature in many cases given that an automatic transmission may
not detererate further for some tens of thousands of miles/kilometers - or
it may become evident later that it is some system feeding the logic of
the transmission that is at fault (say a momentary cut off by the engine
protection system). However - this is a gamble given that your
transmission may also fail outright even after replacing a suspect part
(like valve bodies) and leave you stranded with several hundred's of
dollars in towing assuming it doesn't fail catastrophically and blow fluid
all over a hot exhaust on a dry day and burn your car down and assuming an
upstream problem like an engine protection system cut out doesn't itself
come to damage the transmission.

Were I in your position - I would have a really good shop put a pressure
and temperature gauge on the transmission on a dyno and confirm it is
running within specifications. If not - I would be looking at the torque
converter, supporting drive systems and differential given you've already
done several important internal tranny systems.

Beware - I parked a perfectly good motor home for a couple of years when
the local GMC dealer here in Halifax/Dartmouth (Nova Scotia) swore up and
down that my TH425 transmission was busted - when in fact they had
completely botched a simple filter change so that the filter o-ring was
pitched and was simply drawing air at the pick-up tube in the tranny pan.
With almost exactly the same symptoms you are describing incidentally..

Anyway - I would also have the tranny filter pulled and cut open by
someone who knows their stuff to confirm if there are any larger bits of
clutch, gasket, gasket sealer or metal moving through the system. This is
not a final answer - if there is contamination - you need to know where it
came from and why. There are almost always bits after service unless the
technician is *really* meticulous - and even then when the new parts are
'breaking in' it may appear to be problem when it is in fact normal. In
point of fact - if your filter is perfectly clean at this point you may
want to know where the 'break in' crud is actually going.

If everything is peachy down there according to someone you can *really*
trust - I would be looking at the engine management system given another
experience of mine with my 89' 9000 Turbo automatic which was (finally)
traced back to an ignition fault that saw the engine management system
very briefly cutting in and out when an spark plug grounded, which was
read as a knock by the ECM system. This caused the 9000 to momentarily
buck, eventually violently, under boost and was repeatedly diagnosed to be
a transmission problem instead of the real culprit - a 5 cent o-ring on
the distributor.

Personally - I have moved back to 5 speeds on my SAAB(s) given the
proprietary diagnostics requirements on an already complicated engine and
drive management system. If I could fit one - I would actually prefer an
old 4 speed off an early 99/900 (particularly on a modern turbo).

This is not to say that the automatic is a bad tranny - frankly it was the
best automatic transmission in terms of drive I've run - but because it
was so hard to find anyone to actually correctly diagnose tranny problems
these days, it started to make sense to move back to a 5 speed despite the
learning curve required of the rest of the family to operate it.

Hope it works out - lovely machine and not too expensive to convert to
five speed - all things considered and all avenues exhausted.

--

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