Steppers can't lift X carriage up

! have just finished constructing my first Prusa Mendel Iteration 2.
All the motors X axis/Y axis/Extruder work fine in both directions.
Z motors are OK when lowering the X carriage but struggle to raise it.
My NEMA 17 stepper motors are advertised as 48Ncm which is above the minimum 40Ncm mentioned in various Wikis.
When the Z screwed rods are disconnected from their coupling, the motors rotate freely in both directions.
Logic tells me that it is a mechanical problem, being harder to lift something than lower it.
I checked that the Z smooth rods are parallel and that the X carriage is not skewed horizontally.

Any ideas of where the problem lay would be most welcome.

Simple set of tests:

1) With current applied, you should be unable to turn the motor shafts with your fingers.
2) With no current applied, you should be easily able to turn the whole Z drive setup with your fingers.

It's a pain to cycle the thing manually, but that is one way to catch a rough spot.

You have two motors on the Z axis. If you have set your Vref's to match up with your motors, the Z axis driver gets twice the Vref as the rest. Each Z motor gets half of the current from the driver.