Hasta La Motion!

I’m having (I think) a problem with the motionblur in the roto/rotopaint tool: it’s not smooth, you can see "steps" – I have attached an image to show what I mean.
This problem only started a couple of days ago; and although there are ways around it, I would really much like to correct it at the source. I’ve checked the viewer settings but can find anything weird; and the problem appears even after render. I’ve also obviously checked settings for the roto/rotopaint tool, but you can always see those weird steps.

Any suggestion would be lovely!
Thanks,
Benoit