Unstable X/Y axes
My Prusa appears to have developed a case of the morning after shakes... A couple of prints I've done today came out with distinct "ribbing" up the sides - not ringing, these are more like deliberate ribs like you might see for grip purposes. Looking very closely at the X belt while the hot end should have been making a dead straight travel along the Y-axis, I can see a slight tremor with the X-belt moving back and forth. The movement is tiny - fractions of a mil - but it's visible. I can't see the Y-belt, but the effect is visible on prints along the X axis so presumably it's doing it on that one as well (and for all I know, the Z axis and extruder, although I can't see anything there).
Note that it's inconsistent - it's not happening all the way up the sides.
I'm using Simplify3D and, for a change, I've got the same model sliced with Slic3r (Prusa edition) to set off once the current print finishes, but this looks more like voltage instability or something rather than slicing.
Any ideas anyone?