The variation is slight? Then, most likely it is the lumber. Few 2 x 4's are truely striaght. If you were to span the same three studs in different places from the floor to the ceiling you will probably see the "rocking" change.

Your drywall will span these and the variation will be minimal - you will notice it again when you install base and shoe but they too will make the necessary bends.

Good luck with your basement.

Most studs have a bow in one direction.

If you sight down the length of the stud, the bow (which may only be an 1/8 of an inch) will be come apparent. Make efforts to always have the bow going in same direction; i.e., either in or out on any given wall. This will minimize the variation in the finished wall.

It is also important to only purchase kiln dried wood, and when purchasing it, reject anything that is obviously twisted.

I heard from a builder show to put the bow (crown/bump, etc.) outward from the finished space, and that worked for me when I did my basement. I'd mark all the studs first with a Sharpie.

When a portion of wall or ceiling joist had that "bump" you mention, I took it down with a power planer. That might be overkill, though, but I had the tool available.

Metal studs are the only way to go...I am sick of warped wood.

Water condenses on metal easily, leading to corrosion or mould on adjacent surfaces. So be sure the exterior walls are foamed or covered in foam board and the plate is on XPS.