Steve,
I am just going by what I have seen with those symptoms before, Somehow a hiccup in the decoder, a spike on the track, someone miss typing while setting up a consist and the loco may have been consisted and the way to check that is to read CV 19, the consit address if it is 0 then that is not the problem so look elsewhere.
If CV 19 has any other number then it is in consist and will only respond to movement commands on that number.
Changing CV 19 to 0 will take it out of consist and allow it to run on it's normal number.
It only takes a few seconds to read one CV to find if that is the problem.
Tony
