Vertical seams visible on curved surfaces on bq Prusa i3 Hephestos

Hello good people of the world!

I am in dire need of help. I have a bq Prusa i3 Hephestos with a Diamond Hotend bowden triple extrusion on a Rumba and a heated bed, everything from reprap.me Diamond Kickstarter. Repetier (0.92.9 firmware) + slic3r combo is nice and all is good with color mixing and everything (quite satisfying results, btw) but there's a problem i can't seem to shake:

The X/Y axis motions are not smooth, and wherever there's a vertex in the 3D model, i get a tiny, itsy bitsy small break or stutter, and thus, a little bit of extra, unwanted extrusion. This causes every vertex to be visible, and it's especially annoying in curved surfaces, like circles but not only. Vertical spines or edges go up and down along my prints, right until the polygons are so tiny and vertices almost overlapping, that this problem appears to disappear. Small/high poly objects print nicer.

I first thought i might have a problem with perimeters (inner and outer). I haven't been able to pinpoint the issue to some specific setting. I've tried various perimeter speeds, extrusion, jerk, timing values, but what i get is that straight lines get printed correctly, as opposed to curved surfaces, which get a jagged, wavy outline. I think it's a firmware/electronics issue but i'm at my wits' end.

Please help!

https://dl.dropboxusercontent.com/u/...6/IMG_7767.JPG

Did you managed to figure out what was it?
I believe I have the same issue also with a Hephestos i3, but highly modified.

My controller is a RADSS with RADS128 stepper drivers, so they can produce a quite high and smooth stepper movement.
Still I get this perfectly regular marks in the filament. It happens with any filament and at any speed.

Looks like the extruder stepper doesn't keep a continuous smooth movement but rather have small pauses.

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