When I turn the throttle knob, my locomotive quickly changes speed in large increments.
A very common common problem is the throttle "jumping" 3 or more steps each time the throttle is turned. This simply means the system is not set for 128 Speed Steps in that particular address.
Earlier decoders, and some less expensive decoders, were set up for 28-speed steps or even 14-speed steps; very early command stations were only capable of handling 28 or 14 speed steps. Technology has marched on, and most modern systems now operate using 128 Speed Steps.
When first turned on, Digitrax systems are set to deliver 128 speed steps. To accommodate earlier decoders, the Digitrax command station can be status edited so that a particular mobile decoder address is operated at these coarser values. This operation is conducted with the command station rather than changing values in the mobile decoder.
The "jumping" is occurring because a 128-speed step decoder is being accidentally operated with a coarser control setting.