Had the same issue with our Reese slider and it was caused by the hitch "winding" up when stopping and releasing it's energy when pulling off. I adjusted the trailers brakes and controller to provide more stopping power, less stress "winding" up of the hitch. Also, after stopping I would lightly let off the brake pedal allowing the pressure to slowly come off the hitch preventing the clunk when you first take off and accelerate from a stop.
I did forget to lock the slider once, you will know if it's loose when it slams back or forward, will scare the **** out of you.
Solved all of this with a new truck and a new hitch, B&W Companion and turnover.
