ramps 1.4 one motor on Z axis not working correctly

I have a Ramps 1.4 and one of the motors on the Z axis only turns a little when compaired to the other. And neither one of the movements on either motor is smooth. I switch the wires from one motor to the other, and their place on the ramps board and also switched to a known working driver. I also tried moving the little pyrometer on the driver clockwise and counterclockwise, Still the same result. I was testing them with the LCD with the lowest mm/s , and not with Printrun like I plan on doing tonight. Any help is appriciated...
thanks

One of your motors is either:
1. Faulty (could have been damaged in transit, by being dropped on its shaft, or by something getting in the bearings such as metal filings).
2. Different to the other (they must be the same spec in all respects).

Try each motor individually, and with any luck you will find one doesn't move the amount you have specified it to move, which would indicate it's faulty.