Another possibility, although less likely, is that you have a loose connection to the decoder from the track. If you have pickups from all 8 wheels that really reduces the likelihood of dirt being the cause but all those connections will join at a single point before being wired directly, or via a socket, to the decoder.

So I would check that the wiring is all good and that the decoder is seated in the socket securely (if there is one). Check also that the decoder isn't shorting out against anything, i.e. that it's held securely and not able to move about as the train moves.

Julian

We're up and running again and looking good! Megga scrubbing of the track, wheels, contact cleaner and contact lube. My goodness this one needs pampering, and I thought the wife was bad ! Been running for 10 mins now and I think it just needs a bloody good run in now. Cheers for all your help as usual, the tip here is don't leave a new hornby hst in a cold attic for the winter !

Oh good news John ,lets hope thats it then.:thumbs