If only the loco hit the floor... where was the DCC decoder... In the tender?
It sounds like you verified that the loco itself runs smoothly, but was that testing done with DC (not DCC)? How did you get power to the motor?
If the jerky running only occurs with the tender connected it sounds like there is a broken wire connection or power pick-up issue in the loco or between the loco and tender. Carefully verify that no wires are broken, especially around any connectors.
I have one of those BS units as well that suddenly stopped all running. I suspect a broken wire. But how does one open the darn thing without breaking something?