Prusa i3 Pro Y-axis Layer shift problem

[demolitia]

I've been printing some stuff with my Prusa that came out ok.
But when i try and print stuff that are higher (with more layers) it sometimes shift layer placement.
It's the Y-axis thats the problem (the axis that moves the heat bed).
What might cause this problem?
Thanks for all help! =)

[john]

Could you take a photo for the part you printed and paste it here? It will be helpful.

[vad7]

I agree.
It is very difficult to assembly "Y-axis" correct, especially make "rectangle".
Y-axis design is shaky and is not stable.

[demolitia]

I've checked and measured the Y-axis rectangle and it is good.
I've changed the belt so its not too tight which might have been a problem also.
Printed a test yesterday and it came out perfect. Will test more this weekend and see what happens.
Thanks again!