It is not the decoder.The decoder in the sound value has the same motor control as the Tsumani. You can also access pre -programmed speed curves by setting a few cv's. Or Adjust 2,3,4,5,6. That is it. I can run mine about a scale 3mph without lurching or stalling. Maybe you have a bind in the drive. If you have to use cv 2 to get her going at 2 speed steps .Good sign it is mechanical and not the control of the decoder.
Johnny
Johnny