Jose,
I think there is another clue in your picture. As Jeff says the vertical bar on the left of the display is the beam spot before the sweep starts, but it is not just a narrow vertical line, it seems to be several vertical lines spread over the same horizontal width (about 0.4 div) as the 'jitter' in the sine wave. This looks more like there is some low frequency noise in the X amplifier which could also explain the apparent horizontal jitter in your previous pictures of the calibration signal (I can't find these online any more). A quick test is to put the scope in X-Y mode with no inputs and see if you have a short horizontal line of about 0.4 div instead of a spot.
Regards,
Roger
