I am also having similar troubles. I found this, hopefully someone can help.
Instead of a UM3, I've running a UMS5 with the most recent firmware (6.2.0).
Each time I start a print, the printer runs through the the z-calibration step. 9/10 rimes it will get to the nozzle, stop then continue to the other nozzle, stop, and then continue to push both nozzles up into the head. When it reaches the maximum height and can not push the nozzles up anymore, it will jerk back down and say that the process failed and would abort the print. The rare times it does get past this point, it begins the auto-leveling process but then fails after getting about half-way. It gives me an error saying that the auto-level could not be complete.
I have read on this forum that something hat could produce an EMI could cause this. I put my phone in another room, put my laptop and the computer tower to sleep, and let it run again. Did this two or three times with one of the two above sequences happening. I thought that it could be from my own static (also read on here), so I started it and then left the room -- still had the computers and stuff off and away. It got past the calibration step, but then failed on the leveling step.
We do have a second printer (also UMS5) on the other side of the room. Could this be the problem (although it was never a problem before)?
Is there something I'm missing, can anyone help?