I don't know if it's a common problem, but here's what I found when mine stopped going round. With the rear access panel removed, there is a white plastic enclosure in the bottom left corner. This contains the Remco 5535 motor speed controller pcb. Remove the short edge connector, revealling terminal marked "T1". The terminal connection on the pcb had completely eroded away, cutting off the supply to the motor. This was easily repaired by soldering a length of 15A fuse wire to the remaining track to restore the connection.
Mick G, January 2006