Anyone ever experience your converter (aftermarket) unlocking on its own in 4th between 50-60 mph even when the cruise is set

 

You might wanna see if the PCM is commanding the converter to unlock at those speeds. I dont think the converter it self is the actual problem. And make sure your Brake switch is adjust right because the PCM might see the break switch coming on so its decides to unlock the converter.
Good Luck.