II. Check the rods
There is dirt cumulation on rods during prints. This can cause resistance at running of the printing head. Result is irregular layering. To fix the problem clean up the rods and oil them smooth.
It might happen you will see the printing head is slowing down at several points (locations) while printing (or it will stop/block). This is casued by rods bending. During the time leading rods can bend. Solution is to move the printing head manually over the rods and check/find the place where is the resistance point.