Stepper motor failing. Electrical interference?
Interference is unlikely to disrupt a signal powerful enough to drive a stepper, but the resistance of normal ribbon cable is a likely problem. 
On the other hand, interference from the motor cable could disrupt other circuitry nearby, so some shielding might be called for.

Two experiments:
(1) run both steppers off short cables; they will probably work fine. (If not, give the "far" one its own driver amplifier)

(2) Run thick wire (cross section at least 1mm^2) to the far stepper motor to reduce the resistance in series with the motor. There's not a lot you can do about the extra inductance of the longer cable, but at low speed it's not likely to be such an important factor. This should give reliable operation up to a certain speed; but you may have to reduce the rate on this axis a little compared with the other axes.