I've had my UMO+ for a little over a month, and it had that problem from day one. I tried all the recommended fixes, but I ended up having to replace the Z linear bearings with good quality Misumi ones to completely solve it.
It seems like this problem is very common and may be getting more so. For me the 40USD cost to replace the bearings was well worth it.
See my post here: Z layer error / skipping layers
Try all the fixes on the 3Dverkstan site, and if all else fails, go with the Misumi bearings.
looks like this one http://support.3dverkstan.se/article/23-a-visual-ultimaker-troubleshooting-guide#z-dropEdited by Guest
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