You have 2 choices. 1) Live w/it, or 2) remove the planks and relevel the floor.
Obviously, as you're already experiencing, sanding an uneven floor is, well ..., resulting in an uneven floor. I'm not going to assume anything but I'll assume your planks are solid, through and through. I know of folks removing the planks and using a belt-sander to do the sanding and then relaying them. Heck, if you're going to do that, you'd want to level the floor since the joists are going to be exposed. (The Pro would 'sister' new joists right to the old/existing joists.) Hopefully, the unevenness isn't that drastic. Anyways, you could look into a square drum sander. It may work for you but you're still going to have the low spots to deal with. Depending on the condition of the planks, they too may need to be replaned. And I'm saying rePLANED; not sanded. This may be necessary because, once they relevel the floor via installing new/sistering floor joists, if you don't replane the planks, THEY will be uneven for sure!
You should think long and hard about other 'short cuts'. It may end up causing your uneven floor to look like a real bad refinishing job. My best to ya and hope this helps.