It could be one or more of the following. Dirty track/corrosion, same with wheels and pickups.
Yes, they may look clean. Start with the track.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
Steffen, I have never encountered this phenomenon. If I have problems with dirty track, or dirty pickups, the sound decoder usually just stops. No sound, no motion.
Are you absolutely certain your reset was complete? Did the loco only respond to Address 03 afterward?
I think that, in the end, you will decide that the decoder is defective....but not if both decoders are doing the same thing. If both decoders are doing this, it must be the throttle or the base station. Unfortunately, I can't help any further.
-Crandell