I recently has problems with a Dapol A4 which ran perfectly on DC power but which, after conversion to DCC, shorted-out on curves whenever run backwards! I found that the motor brush wires were shorting-out to the 3rd set of tender wheels because the brush-capacitor wires were too close to the wheels. Once I had spotted this, I carefully bent the wires well clear of the wheels and the problem was completely solved!
Doug