Solved Z motor voltage?

I'm using the Robo R1+ with lead screw upgrade. I have recently been hearing a strange clicking sound, one click every few seconds, particularly when mesh bed leveling is activated and the z motors are moving steadily. After some troubleshooting I've determined it was coming from both Z motors. I think they were skipping steps, resulting in very poor looking prints. I decided to have a look at my driver voltages. First of all both my z motors are hooked up to the same driver. I checked the voltage going to that driver and it was around 0.55v. Based on what I've been reading, 0.4v to 0.6v is an acceptable range for EACH motor. Meaning if both motors were on the same driver, wouldn't that mean I would set it to roughly 0.8 - 1.2v? At least that's what I read in more than one place. To eliminate the skipping, I increased it to 0.95v, and my z motors went nuts when I tried to manually raise/lower them with Repetier. They seemed to pause every other step and click like crazy. I expected that to happen at a much lower voltage! Then I decreased the voltage down to 0.65v, and the clicking went away and the z motors appear to be functioning fine now. I don't think it matters, but I recently replaced my hexagon extruder with a E3Dv6 clone. My questions are:

1. Is there a reason this clicking would have just happened on its own without touching the voltage?
2. What voltage range should my Z motor driver be at, considering both motors are hooked up to the same driver?
3. Can someone explain why increasing the voltage (to what I thought was an acceptable range) caused the motors to behave so strangely?

I'll add that during printing, the motors don't get hot at all. They barely get warm.
Thank you!

 

1. But if increasing the voltage on the driver got rid of the clicking, do I really need to replace the drive

2. Also, I have a total of 4 drivers on my ramps board. Both z motors are going to the same driver. Wouldn't that mean they are sharing a driver? I bought the printer used, so is it possible the old user might have replaced the 2 drivers with 1? If this is the case, how is it possible both could function at only 0.65v?