RX100 may be the wrong ratio for modern road shifter.

If you look at the actuator arm on the old derailleur, you will see that the arm is approximately the same length on each side of the pivot screw. The new Shimano road ratio results in an actuator arm that is approximately 1/2 as long on the cable side as on the cage side.

It would appear (from many recient postings here about this problem) that Shimano was using something more like the current MTB ratio back then.

Try adjusting the high limit screw first but if that changes nothing, you will need a new derailleur.