[valiant67]

Limp mode from the moment the car starts has a good chance of being an electrical/sensor problem.
Limp mode that happens after a lurch is likely because of a low pressure situation. Low pressure causes hard shifting. The revised TCM was more tolerant of low pressure and adapted accordingly resulting in less hard stuffing, bump stops and limp modes.

 

[valiant67]

Probably be worth getting it on a scanner that will read the transmission codes so you know what you're dealing with.
Otherwise you could work on all the electrical connectors, get an updated TCM and new ATF+4 in it and still be in the same situation.

 

[valiant67]

The symptom you described of having to drive the car a certain way to avoid limp mode is pretty consistent with the issues the upgraded TCM was designed to fix. I suspect you've got a low pressure fluid situation which leads o the slow reaction and the eventual drop into limp mode. Based on your description I don't think the issue is purely electronic. I would get fresh fluid and a revised controller on is as soon as you can. Fresh ATF+4 flows better than old fluid if it's got ATF+3 in it. If it's already got ATF+4 then the problem may be more severe.

You are describing the exact behavior I had with my 1992 New Yorker Salon. But there were times in traffic I could not avoid braking then accelerating without waiting. Eventually I got good at restarting the car while rolling. The ATF+4 fixed the problem of it never going back into limp mode but the shifts were rough and the error codes pointed to a low pressure in some circuit. New updated junkyard TCM and it shifted perfectly afterwards.