Lead screw synchronisation problem (1 reply)

I've assembled a Mendel90 but am using a RAMPS 1.4 and the standard fork of Marlin.
NEMA17 78 Oz-in CNC stepper motor stepping motor/1.8A on the lead screws - Model Number:17HS8401
I've wired them in series as in the assembly instructions (2 wires from the board go to one motor, 2 to the other then they're connected together)

In pronterface I can home the z axis as well as move it up and down as much as I like. If the motors are off I can easily turn the lead screws with my fingers.
When I try and centre them with "G1X0Y0Z100F30000" the left hand leadscrew drives down but the right sticks and the motor makes a grating noise (I can see that the motor drive shaft is not turning, it is not just that the coupler is slipping).
I have an lcd controller hooked that I can control the axies with. If I use the LCD to move the Z axis I get the same jamming problem, but it seems to alternate from one leadscrew to the other.

Any ideas?
I'm stumped... I tried turning up the pot on the Z stepper driver but no change. The Z stepper driver isn't hot to the touch either.