sloping back along Y-Axis ?

I am 90% there... prints are starting to become smoother but the Y-Axis is sloping back along with each layer.

I've attached an image of a calibration cube I just printed.

I believe I need to adjust the Y-Axis to compensate for this drifting backwards. X-Axis is great and my steps along the Z-axis height are fine too.

so what's the best way to adjust this Y-axis problem... in the calibration.h file or is there a physical adjustment I can make to the printer itself
I'm on a Prusa i3.

CalibrationCube Image

Edited 4 time(s). Last edit at 08/13/2013 08:44AM by Mickman.

Looks like lost steps. Try a touch more current on the stepper driver. Maybe a slipping pulley. Not a calibration issue.

Edited 1 time(s). Last edit at 08/13/2013 04:13PM by Dale Dunn.

Everytime I've had this issue it's been related to belt tension. I would tighten the belt in that axis and re-try the print. For a quicker test, try the 0.5mm thin wall cube found on the Wiki: [reprap.org]

It may help to see the PTPG [www.reprap.org]
with a list of the usual suspects.

Another thing to check if the other suggestions check out ok, is to make sure your z-axis is perpendicular to the print bed

I'm checking the belt tension.. I think this could be the culprit.. not certain though as I can't perform another print till I work out why my hetbed has stopped heating