1. Some decoders are prone to run away if DC mode is enabled in CV29 (they mistake a corrupted signal as DC mode). Turn off DC mode when on a DCC layout. click here for instructions on CV29
2. If you have Wireless Cabs in your NCE DCC system are not version 1.5B OR your RB02 is not upgraded to version 2.0 or higher you can get a situation where one throttle suddenly goes to speed step 128. You need an RB02 upgrade. go here for more information: https://ncedcc.zendesk.com/hc/en-us/articles/205069179-Read-First-Wireless-Version-Troubleshooting-Summary
3. If your track your track bus wiring has any single section longer than 30 feet your track bus may be conducive to electrical spikes. A decoder can be confused and run away. You need to fit snubbers as per page 10 of the Power Pro manual or use the links below. You may need to reconfigure your bus wiring as per the best-practice recommendations in the same references. For more snubber / RC filter information go here : https://ncedcc.zendesk.com/hc/en-us/articles/204128735-Snubber-RC-Filter
4. You have the same loco selected by multiple throttles and the throttles have two different speed settings.
5. The batteries in your wireless cab are near dead and the cab switching off while in use.
6. Wireless cab shutting off: Using a wireless cab and not touching any buttons for 5 minutes while a train is running. The cab times out and shuts off, so your cab has no control when you go to stop the train. press any button or jog the speed wheel every 5 minutes to keep the throttle awake. You can also disable the auto off completely but then you will need to shut the cab off manually after an operating session.
7. Some decoders have an option to remember the last known speed command after a power interruption. This improves dirty track performance. But if you pick up a moving loco from the track and then replace it some days later or on a different layout, it will start immediately at the last known speed.