Bought a Geeetech i3! Great value! Except it came with a bodgy Z axis drive. The threaded rods were bent. Leading to the ripples/ wobbles associated with such a flaw. I bought new screws for it they were straight but unfortunately their thread was too coarse for the steppers to hold the weight of the x Axis. It kept on going out of true. I looked around for another answer to the problem. There are a few of these guides on thingiverse. Bernbout's thing is the basis for this one, it allows the bent rods to move more freely without affecting the z axis location. Mine is a little thinner so you don't need to lift the table supports. Bernbout did the "hard yards" and should be congratulated for his efforts.
I include this part in any i3 that I make or rebuild even if the screws do not seem to be bent.