strange shift in print

Hi, I'm about to use my Hadron for larger prints and run in shifts during the Print and don't know how to fix them. They occur in the X- and Y-plane.
when they occur, i don't hear strange noises, or something, I checkt the screw on the sprocket wheel, the fixture of the belt on extruder wagon and bed wagon,
taped the mirror down on the heatbed, so it really can't shift. I don't know what else could be wrong.
I read in "Mike's ORDish Bot build" about reducing the current of the stepper driver.
But if i go down with the reverence voltage, the motors won't move, just wiggle and grind. (rev V about 0.15V on printrboard)
I printed this part two times and every time it shifted nearly at the same layer (+-2) but one time only in y and the other time in x and y in one layer (and for the same length => 45 degree o.O strange)
hope someone can help, everything works just fine, but it shifts and ruins every print...
when they occur, i don't hear strange noises, or something, I checkt the screw on the sprocket wheel, the fixture of the belt on extruder wagon and bed wagon,
taped the mirror down on the heatbed, so it really can't shift. I don't know what else could be wrong.
I read in "Mike's ORDish Bot build" about reducing the current of the stepper driver.
But if i go down with the reverence voltage, the motors won't move, just wiggle and grind. (rev V about 0.15V on printrboard)
I printed this part two times and every time it shifted nearly at the same layer (+-2) but one time only in y and the other time in x and y in one layer (and for the same length => 45 degree o.O strange)
hope someone can help, everything works just fine, but it shifts and ruins every print...