Weird Layer Shifting on a RAMPS conversion

sorry to keep pestering you lot, but i have have the weirdest issue, i recently converted to ramps.

I followed the instructions exactly, setup the firmware and sorted the Z height.
on every single print, on every 5th layer the print moves across about 1 mm.

when printing a cube, it looks like a flight of stairs, every layer is bang on except layer 5, 10, 15... which it shifts

i dont even know where to begin to look to address this issue, any advice would be great