Chrysler Repair: 94 LHS transmission goes into limp mode, transmission oil cooler, computer box

Question
Roland...
this will test your skills....it has tested the best so far....and still the problem exisis.
94 LHS, 3.5 engine. 69,000 original miles...I am the only owner....well maintained car.
Problem....
started a year ago....when the engine is warm....(in Guam...it's always warm!) after driving some time....20 minutes....the transmission jumps into 2nd gear. I had to shift into "N"...coast and shut off the engine, restart and shift into "D" and go on my merry way until a few minutes later...the same thing happens.
I went to the dealership...."told...you need a TCM....replaced.....week later...same problem...return to the dealer....you need a transmission rebuild..."I say" you told me last week I needed the TCM....now it's a rebuild!? I talk to another tech and another shop...I am told...the problem is electrical, you don't need a rebuild. OK...the computer box inside the transmission is replaced...(The transmission pan was removed to gain access for this replacement....the entire transmission was not removed.) Car is fine for a few days....problem again....no codes I should also add.
I then buy a used 96 Intrepid...same engine/tranny....I did not test drive the car....rear end damage....the donner car has an auto stick...my car does not...still the shop said...no problem  it will work.
Transmission replaced....car runs fine for ten days! Then....jumps into limp mode/2nd gear again! Back to shop...shop says...your oil is getting hot....we live in Guam....install an external transmission oil cooler. OK...installed....temps drops by 30 degrees. Problem goes away....for a week!
Any ideas? No codes stored Roland. Shop says...rebuild the tranny...the used one you bought apparently was on its last leg.
When I go driving Roland.....start up is fine, cruising is fine until 30 minutes later when I stop the car and do some errands and return...start up and drive off.....then it goes into 2nd gear. Also...when I am driving down the road...if I were to shift into 2nd gear from drive or 1st...it does not work...stays in "D." If from a stand still I shift into 2nd...the car stays into 2nd while driving until I shift to "D" and the it shifts fine into "D."
Any ideas?
Thank you!

Answer
Hi Ben,
When you say there are no codes, do you mean that you have tried via the ignition key "on-off-on-off-on and leave on" approach or do you mean that you plugged a Diagnostic Readout Box (DRB II) into blue data link connector under the dash just to the right of the steering column and readout the codes specific to the transmission? The shop has to have a DRB II and the associated '94 cartridge to access the codes for the trans, otherwise you won't get codes from the check engine light approach.
The fact that it is going into limp mode means that the TCM will have stored a fault code, because that is part of the process of setting it into limp mode. I haven't heard of this "truly no code" being related to a failed TCM but that could be the case if you can't get a code out using the DRB II.
Your last paragraph was a bit unusual as to the manual shifting manuoeuvers, which makes me wonder why you not just start in second gear all the time? I suspect that will not work over the long haul and instead when it is ready to  go into limp it won't shift from 2nd to D but rather refuse to shift.
Another possibility, although an outside chance, is that the gear shift cable is not adjusted properly. Do you have the floor shift or the column shift? I can type in the steps for one or the other, or xerox and postal mail you the pages from the manual that also have one illustration that may be helpful.
Because you have tried two transmissions and replaced the solenoid pack in the first, maybe a TCM is the answer. Did you try using the TCM from the '96 in your '94? If so and you still don't get codes, then the chances of both TCM's failing with "no codes" makes me think that they aren't accessing the TCM properly (DRB II/94 cartridge) to get codes.
Those are my thoughts on the situation. Let me know what is your reaction or what happens with trying one of these alternatives.
Roland