Well, the only two parts that typically would cause a runaway are the R2LC or the DCDR in this locomotive. The tender components are strictly the sound, they wouldn't cause what you see. This is the DCDR, it's the only other component I can imagine that could be an issue with those symptoms.
Obviously, I'd be checking continuity for all the wiring between various components at this stage, but if that fails, you'll likely have to try replacing the DCDR. The sad part is, if you lived in the US, I have a drawer full of those.
