Does a capacitor charges smooth, or in stairs?

[Labrat101]

This time error showing a negative back step is only a few pixels.  I would think this is due to time problem between the speed of the screen video timing and the main time base are out of sync by a few pixels.
Could be a component delay or a small error in the Dac division..
If its real you have found a way to travel back in time  which is really good .
I think you are pushing your scope .
And with all electronics there will be one part that may be not 100% up to spec.
I would check your video memory  maybe getting hot..
Have fun
RNS