Bachmann Dynamis

I have started on the DCC route and have bought a Bachmann Dynamis. I have chipped 5 locos so far and addressed them. I have linked all 4 mainline tracks to the command box via two on-off switches, so that a further "always on" feed goes to a programming track.

Everything seems fine and I have checked the track with a DC system to ensure that there are no short circuits. However, when I use any of the tracks, after a lap everything grinds to a halt - the transmit/receive indicator falls to zero and everything stops. No short is detected on the handset, but I have to press "stop" twice to get things going again. Do you think that this may be caused by dirty track, so that the command station isn't receiving a feedback signal - or it just faulty?