Also check and recheck the calibration. On umo+ mxl belts at some speeds it's easier to see even a small error in calibration (I mean even a half millimeter).
On the long run. Print and install twisterblocks by chopmeister and if you really care about precision ditch the bad umo/umo+ mxl pulleys and upgrade to gt2 (the ones um2 uses). I changed my 3 machines to that (two on the assembly day) and it's just fantastic (slightly better but noticeable).
About t gt2 upgrade on umo/umo+
https://ultimaker.com/en/community/15874-mxl-vs-gt2-print-results
A better side blocks:
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IRobertI 521
You probably have a bit of backlash caused by loose belts:
http://support.3dverkstan.se/article/23-a-visual-ultimaker-troubleshooting-guide#circles (UM2 is pictured but it applies to your UMO as well)
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