1984 deville trans problem?

I have a problem, could be the trans?  the car acts fine driving on the city streets, shifts/downshifts fine, but if I drive on the highway at 65-70mph for a while when I get off and stop for the first time the car will buck, like it is not downshifting or something, it has even died before, starts right back up, like I said I only have this problem after driving at speed for a while.  I have not had the car for long, trand fluid is new, changed it right away to see if the problew went away, it did not, any ideas?

I don't want to beat a dead horse.  Just try disconnecting the wiring harness that runs into your tranny.   Tell me what happens then we can go from there.