So i builded myself a Prusa i3 and i had a lot of problem with the x-axis, the default x-carriage that can be found on the Prusa github (https://github.com/josefprusa/Prusa3/tree/master/box_frame) have 3 bearing, 1 for the upper bar and 2 for the lower. All i did was modify the original scad adding a 4 bearing, now all bar have 2 and at least for me worked like a charm. In the file you can find the 3 standard version of the carriage (30,50,80) for Gt2 and T2.5 belt, i added the .scad if anyone needed to change something. FYI, this carriage can probably cause some clearance problem with some extruder, right now im using this one http://www.thingiverse.com/thing:65939 and it works greate.