Gimp - jagged pencil and brush strokes [SOLVED]

I am using Gimp 2.8.14 on Ubuntu 14.04 and a Wacom tablet (Intuos 3 6x8).

My problem is that I cannot make a smooth stroke even when I check the Smooth stroke in the Tool Options panel. When I do that the problem is less evident, but is still there.

I believe the tablet is not the problem because in Krita the problem does not appear.

Here is the link if you cannot see it:

https://drive.google.com/file/d/0B7mcZJoyvCM0SHVIeFVuWEJiU2M/view?pli=1

I have already uninstalled Gimp and installed it again and the problem persists.

Can you please give some directions to solve the problem?

Thank you, very much.