Need some help with my new 3d printer

I've just assembled it. But i think i must have done something wrong. Take a look on this video and especially notice what happens after 1:30. Its like the left Z axis begins to jam and it never stops once it reaches the bottom. and then it goes bananas.

YouTube video

What have i done wrong ? I've tried tighening the coupling on the left side, but it is fully tightened.

Its a Geeetech I3 Pro B btw.

Sounds like the vertical linear rods are not properly aligned and cause binding.

The left Z stepper is loosing steps, either through excessive friction on the left leadscrew or insufficient current through the windings. Are both z steppers plugged into the same stepper driver? Is the z stepper driver getting too hot to touch?

Does it have the same issues if you manually reset the x carriage level at z=0 and drive the x carriage up to Z max?

Can you release the left hand leadscrew nut from the x carriage and manually run it up and down the leadscrew without force? It should spin freely.