I have found all my old 32 speed step TMCC locomotives preform like this with a Legacy controller, so I just keep using my Cab-1 controller for those locomotives.
I have this same locomotive, but my problem is that the longer it's running on the layout, the slower it gets. I thought maybe it was getting to hot, but the shell is still stone cold to the touch. And then after a while it doesn't even have the strength to pull a train around O54 curves. Not sure if replacing the guts with ERR cruse would help or not.