The X axis shifts

Hai ,
Just received my Vertex nano, and everything goes well during construction, good manual, all parts present, in short, nothing to complain about
My first print started as it should be no problems, but after 2 or 3 layers of printing, the x axis shifts + - 4 mm, after that the printing is excellent 8 mm high nothing happens, then a shift of the X axis again, then everything okay again
I can’t find anything where possibly the X axis blocks, the belt is fairly tight, no unwanted noises, only the buzzing of the fans
Who can help me please


Please check that the pulleys at the axis stepper motor is really tight. If it is not really tight, it can occasionally slip.