I was involved in model railroading when Digital Command Control (DCC) first came out. For most inexpensive locomotives these DCC systems worked fine but for the more expensive loco's, with better quality motors, they tended to exhibit a similar problem, in some cases the engine would pulse but more common was a buzzing noise coming from the motor. The problem was that there was a constant voltage being applied to the rails. To get a DC motor to act like a variable voltage the signal was chopped into pulses of power so in effect the motor sees full voltage for a short period and no power. The faster you go the bigger the pulse width. The expensive motor were of such a quality that they would exhibit the pulsing or buzzing as the motor are being turned on and off at a high frequency, whereas the cheaper motors could not react fast enough to show the pulsing. The fact that Hornby are heavily involved in model railroading leads me to believe that they used a similar system for their Scalextric Digital System and we are experiencing similar problem to the early DCC systems. If this is the problem then I am afraid there is not much we can do about it!