acceleration control not working

[Alvonian]

I recently bought the Bachmann Peppercorn A2 "Irish Elegance" and got it fitted with a Hornby decoder. I programmed acceleration and deceleration which worked perfectly. I ran the engine in as described in the accompanying leaflet.

Today, after removing

 

it to attach its extra details, it ran erratically, stopping dead on points which it had no problem with before.

I have tried programming it again, but it does not accept either the acceleration or deceleration commands. It starts like a bat out of Hell

 

and takes 2 or 3 seconds to stop after the controller has been turned off, and then stops dead.

I have a Select Controller and all the other locos are running perfectly.

What have I done?

Any advice gratefully received.