You should really calibrate the stepper drivers. Mine were set to max, which under normal conditions would mean the motors could skip steps due to internal thermal shut down and overheating the steppers to 80c+. Are your stepper IC's sinked? Do you cool the ramps and the stepper drivers? I would check out this link and tune the system properly. I looked for the link on reprap wiki, but the pics seem to be gone, I have the pic from the reprap wiki on my blog build notes. Try this [engineerd3d.ddns.net] Towards the end you will see how it works. I would suggest about .35v instead of .425v that I have, my steppers run at high temp because I went overboard with the amperage, I am going to re-tune them to about .4v.
What I am saying is that the delta design lends itself to small binds in the movement, especially in the extreme, where I suspect the forces are highest, couple that with a thermally overloaded driver that is supplying too much current and possibly a small bind in the movement and you have the makings of missed steps on a single axis.
What I am saying is that the delta design lends itself to small binds in the movement, especially in the extreme, where I suspect the forces are highest, couple that with a thermally overloaded driver that is supplying too much current and possibly a small bind in the movement and you have the makings of missed steps on a single axis.