Check the frame for play, loose belts, loose pulleys, loose bearings, loose joints in the arms. Check the heck out of it B)
It could also be a plastic flow issue. If your hobbed bolt was not built properly and is eccentric it can cause this, but I doubt it.
It may be worth checking if you've tried everything and have run out of things to check.
If it's a plastic feed issue you'll get a wave every X mm of plastic that goes through the extruder.
If it's a mechanical issure I'd expect it to show every X mm of z height.
Thus if it is a plastic feed issue the waves will be closer together where the layers are wider and require more plastic, and further appart on taller thinner objects.
It could also be a plastic flow issue. If your hobbed bolt was not built properly and is eccentric it can cause this, but I doubt it.
It may be worth checking if you've tried everything and have run out of things to check.
If it's a plastic feed issue you'll get a wave every X mm of plastic that goes through the extruder.
If it's a mechanical issure I'd expect it to show every X mm of z height.
Thus if it is a plastic feed issue the waves will be closer together where the layers are wider and require more plastic, and further appart on taller thinner objects.