Oh dear, I believe I've been the victim of a 'good' idea I had a year ago. When putting Chwarel Bach together it seemed like a good idea to ballast with a polyfiller, sand and powder paint mix. It certainly gave a quick result. The first time I used the layout in anger at the Corris model railway show all worked remarkably well. Since then it's sat upstairs in my playroom, pressed into service from time to time as a test track or somewhere to let completed engines run in by lapping the circuit. A creeping and increasing deterioration in the running quality began to make itself felt. After spending a lot of time looking at both loco mechanisms and the track I came to the conclusion that the problem was caused by the ballast. It seems that as well as the usual amount of airborne dust, the pollyfiller is also contributing to a fine layer that inhibits efficient collection of current from the rails. There may be issues with a fine layer creeping into the gap between fishplate and rail further contributing to the embuggerment. I think cleaning followed by sealing the surface of the ballast with dilute PVA is the way to go; meantime have a pretty picture of my latest loco to emerge from the paintshop on the offending length.
