What's good to clean it with?

My ho scale electric bachman train has been doing great for a long time. I have about 100 feet of track that encircles the room. I recently bought a steam locomotive with smoke and it ran fine on my DCC system. However recently both the deisel locomotives and steam loco is running like it is not getting a good connection, slowing and speeding up constantly in a jerky type motion. I took out sections of the track and made a little circle of track but it still did it. My diesels are DCC compatable but the steam locomotive is not. However it ran just fine on the system for at least a week after buying it. Any suggestions?? Please advise if possible. David