Y-axis displacements

Helllo everyone!
I've finished assemblying my Mendel Tricolour couple months ago and didn't face with this problem until last few days.

Like you can see on the picture, there some displacements on the Y-axis. These shifts take place while printing different models at different speeds.
Even trying same G-code twice, or more, ends up with shifts that could be higher or lower or in different numbers.

I've checked the Y-belt, tighten it up several times and displacements are still there.

Any advices?

Have you checked the pulley on the y axis to make sure it is tight?

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If you measure it and its at an even interval, then it's axis wobble. If its not then the pulley slipping is a good place to start. (I had this very issue.)

or maybe binding somewhere?

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Waitaki 3D Printer

Well, I've listened to the pulley advice, disassembled it, added a little glue under the pulley, and after I assembled it againg the printing was just perfect for first three models.
But on the fourth try everything was back to its previous state.

Are your motor shafts all round or do they have flat places on the last third or so of the shaft?

Thank you all, guys, with your help I solved the problem and got some nice results.
First guess was for bindings and after I removed it, the problem was partly solved.
Then I found out that Y-belt is displaced by some angle if compare front and rear parts of Mendel. So I reassembled whole Y-system and everything went fine.

Again thanks all and sorry for late answer.