[UM3E] Mysterious Belt Problem

[mirobane 4]

Hi there, is been a long time 🙂

im back again with a new ridle with a UM3.

so wath' is the problem ? yes, let me in 
 

for explain simple, there is "slipping" on Y belt in the machine while X travels.

 

for be more simple, there is the video in "slo mo"

https://photos.app.goo.gl/ef7o16kRiKN8W18d8

i tried re align all the pulleys and re tigthen everything but nothing do it.

 

BTW, it hapens one everytime machine starts rigth after a homing but after it no more hapens during stress test or other movement of the XY square.

can it be a electronical problem ?