I know this is a really old post but I purchased what I assume is the same Prusa i3 clone off eBay and was having the same problem with the juddering x axis. I believe I may have solved the problem after having a play about with it tonight. The driver chips you mentioned had the Vref set as 0.8 volts and I believe this is way too low. After adjusting it to 1.6 volts the motion seems much smootherby Craigk47 - Prusa i3 and variants