SOLUTIONS (although I don´t quite know what to do with them)
Here´s a picture of what happens to the servo power rail during A shake with only one of them hooked:
I can show you 5 different shakes and all 5 scope waves look exactly the same. WEIRD.
Here´s the same, but with both servos hooked up:
Same thing here: every single shake draws the same contour on the screen. At this point I thought it was easy to figure out: Bad power supply. So I switched it to the lead acid 12VDC 4Ah. This next capture shows different PSU for logic and servos, with their commons tied together:
You can see the noise in sync with the servo commands.
So, even though the dips in the power rail dissapeared, the shake didn´t. It is now less often, but still there. So, it is safe to say that I had two overlapping problems: one that was bad power (just like Dan M. and KingsJester89 predicted - my deepest admiration to them!!) and another one, as of yet unknown.