When you did all the other stuff, did you put bulbs in the taillights? It's way back in the mists of time for me now, but I remember my Dart would occasionally do the same thing, and it meant A) Blown bulb B)Corroded socket or C) Bad ground.
No blown bulbs, all connections are clean and have been dielectric greased, all ground related are connected and in good condition.
It was all fine for the passed few weeks since I installed everything and then out of the blue this problem started and nothing has been done recently to cause it
