What DCC system are you using? This is a classic symptom of the same address selected on more than one throttle, so one is telling it to stop, the other is telling it to go.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
I have an HO Loco with LOKSOUND installed. The Loco was running fine in the past but currently has problems. It has a address of 5005, when I run it it will throttle down in several spots on the layout then come back up and run to the next throttle down spot. It is not shorting. I did a CV8 reset and it seemed to run pretty much OK using address 3.