my WAG: if the spindle drive requires a +/- 10v signal to reach max rpm in either rotation, perhaps the voltage is inadequate in one direction. I don't know if that means the power supply for that circuit is perhaps failing, or whether there would be reason for a resistance to have entered that circuit.
There may also be parameters in the spindle drive that would or could be used to fine tune the drive in an attempt to match the signal voltage to the exact motor rpm. If the motor runs open loop (no feedback), it is possible that someone may have fussed with this parameter to try to get full rpm out of the drive at +/-10v.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)