Z dimension changes when I fly up/down

I have created a flying carpet mob. Unfortunately when I fly up or down my Z dimension changes by itself making the mob moving to the left/right. X dimension varies between 2 similar values (strange, but does not look bad), Y dimension goes up (obviously), Z dimension changes slowly like it is moving to left.

Here is the main procedure I use for making the mob flying:

Do you have any ideas how to keep Z dimensions fixed?

Thanks for help!