Couple of things immediately spring to mind...
- Misaligned motor
- Eccentric motor/leadscrew driving wheel/gear
- Dodgy leadscrew
A poorly aligned motor without a sufficiently forgiving coupling seems like the most likely culprit to me, especially if the error is indeed occurring once per screw revolution. Try cutting a bunch of straight lines lines parallel with the suspect screw (so only one axis is doing any work) of various lengths, and see how close the actual length comes to the commanded length. This should help identify the problem axis if you didn't already know.
You're not using gears to couple the motors to the leadscrews, are you? How about timing belts or similar? If they were mounted eccentric you'd get periodic errors, I reckon.
It is possible that the leadscrew could be damaged but I don't know whether a screw that is bent enough to cause problems yet straight enough to run nicely in a CNC cycle would cause this sort of error. It would be once per cycle periodic though, however it manifested. The severity of the error would likely differ depending on how far along the axix the nut was at the time. Are the ripples the same size regardless of how far along the axis thes bed was during a cut?
You could try sticking in a new leadscrew, though that may end up requiring some new bearings and a little bit of engineering. I've got some new trapezoidal screws for my proxxon KT150 table, but once i'd turned down the threads to get a smooth OD for bearings it'll only be 8mm dia instead of the current screw's 10mm OD. I imagine the KT70 screws are similar, though at least the nuts on the KT70 are so chronically prone to backlash! Possibly you'd need to get some acme screw and make a new nut; no idea how easy it is to get hold of trapezoidal thread leadscrews in the US.
Doesn't sound like a gib issue to me, but I could very well be wrong. Are the ways well oiled? I'd expect a dry way/overtightened gib strip to manifest as random stick/slip rather than a relatively steady 1-twitch-per-rev, though. As you changed gib tightness without any effect, I'd say that probably rules them out as a cause.