Few news and a question about backslash.
After having repared my hotend soldering a new thermistor to replace the one broken.
I had thightened the X and Y axis and I got more problems with the UU bearers. They block the move.
Is that backslash? Will it be better if I replace the MXL PLA pulleys by aluminium ones? Do I have to replace all my T2.5 belts and pulleys by GT2 kit?
What do you mean by this:
They block the move.
One thing I’ve run into with LMxUU’s is when two bearings are used on the same rod, they can bind against each other. Try loosening the bearing holders and sliding the carriage back and forth until they settle in, then re-tighten.
@Michiel_Haisma , I mean X or Y bed are blocked during the move. That leads to gap wider than 5mm between 2 layers.
I unthighten belts. Hope it will fix the problem.
Worst, I have problem with my leveling. Z home is not the same on all the plate and I did level a round one and 2 axes but both disagree on what is correct. 
Sounds like you’re motors are skipping steps. This can be from added friction, or from over/under driven motors. I usually dial the gain down until it can’t move the axis at all, then slowly dial it back up till it runs well, and give it an extra pinch of power. Printing will stress it more than just moving around a few times. Over driving will cause the driver to overheat and skip steps. Hope that helps.
hmmm motors skipping steps. http://3roomlabs.blogspot.sg/2013/03/rsenser4-experiments.html … this could be interesting read for you