Unwanted seam appearing in prints on FFCP

[Horia]

After a lot of tests and calibration we found out that there is inconsistent movement to the X and Y rods.
This could be because of bearings or belts. Don't know yet.

Edit: Saw on a lot of forums that the movement should be "smooth" for the carriage but we can't measure that. Probably we have to disassemble each component to find out what makes the hiccups.

If anyone encountered the problem and resolved it please help.