I had the same exact problem about 2 years ago in my 'Tique. I was getting a code from the EEC-IV that my torque converter was slipping.
and the fluid was starting to burn a little. I drained the fluid and replaced it with cheapo wal-mart Dec/Mec III. Cleared the codes,ran it for a while, it ran fine, then it cam back big time slamming in and out of gears. Changed the fluid again (added a bottle of Slick 50 high milliage tranny additive, don't flame me, it worked) and it has been running pretty good ever sense. No more codes, and no more dropping out of gear at highway speed.
The only problem I still have is that from a stop it takes a little bit for the tranny to kick into gear, and when it is really hot out, the engine tends to die if not in neutral at stop lights. The car is not worth a new tranny so I will nurse it to the grave.
