Delta Pi - Directional Z skewing?

My first attempt to print a large object is showing this really strange effect.
It would seem that depending which direction the print-head is travelling the Z axis will move up or down. Any ideas how to fix this or what is the cause? Is it more likely to be a defect in the carriages or would it more likely be that the arms are not moving smoothly enough? (They seem like butter) Or maybe the belt tension is off?

Anyone who has fixed this kind of problem on a delta before, I would really appreciate some feedback

HA, ok, I feel sheepish now.. one of the screws on the arms assemblies had shaken loose allowing the arm to swing slightly away from the carriage while still appearing attached.
Re-calibrating now to see if all is well.