@Didier, in your video it looks like the router housing clamp is too tight.
in summary, you want the clasp closed, but you want the router to easily move ‘up and down’ (ie: deeper/shallower cuts) in the housing without being able to move in any other direction.
you will want to clean/polish as best you can, the router housing and the router barrel. The smoother their surfaces are, the more pressure you can apply with the housing, keeping the router firmly in place while still allowing the router to move when asked to.
also check the wiki for some other tips to keep the router height adjustment screw in place and precise in its movement.
hope this helps,