Nylon rollers steel stems
That later build date matters for this specific problem: doors from that era came factory-fitted with torsion spring systems and nylon rollers on steel stems, not the older extension-spring-and-steel-roller setups you still find on houses built in the 1960s and 70s. Nylon rollers run quieter and smoother, but after about twenty years the stems wear and the roller can walk out of the track groove under an uneven pull, especially if one cable has stretched more than the other.
