I had a similar problem and this post helped me figure things out: http://forums.reprap.org/read.php?4,405576,459583#msg-459583 Also in your host software, be sure to set whether your home positions for each axis are at maximums or minimums. Having those wrong in conjunction with an incorrect endstop configuration were causing me a bunch of grief with real unpredictable printing behavior. Goodby roto - Prusa i3 and variants