Yes, it looks like you are stuck sourcing a replacement power unit from a pre 1995 car (according to the parts catalog).
Since your EPS light comes on when the rack runs up against the limits on the right or left side, it sounds like the error condition might be caused by the rack motor exceeding the current limits within the control unit. The current sensor is in the power unit, not the control unit which is why switching the power unit may be fixing the problem - if the problem is with the current sensor. The current sensor could be a solid state device or it could be as simple as a shunt resistor with the control unit monitoring the voltage drop across the shunt. More voltage drop meaning more current. 'If' the current sensor is a simple shunt, heating caused by the motor current may have caused deterioration of the solder joints meaning a higher effective resistance for the shunt. The higher resistance will cause a higher sensor output voltage for the same current which may exceed the error setting in the control unit. It is possible that you could have this 'drift' in the sensor; but, the sensor still tests out OK. If Brian K checked over your EPS Power Unit and couldn't find a problem, it might be worthwhile asking him whether he was able to check and confirm the calibration of the current sensor.