X axis is sliding ?

Hi all!
i'm from italy and i've got R1 since october 2013
Until 10 days ago, we (me and the R1) got a good tune together, also thank to this forum help
so, my first msg is about the tune out
The last 3 printing are like the pics
As you can see, only along X axis
The belt seems to be at correct tension, the pulley screws are tight and checked often
I tried a lot of printing parameters, finding here, like temps, speed, layers specs, also tried to rotate the model 90 and 45 deg, the trouble still remain

Any suggestion ?

thanks in advance

 

thanks
i'll try to check it again

 

Its gotta be. It needs to be tight like a guitar string...but not hinder movement.