How "warped and uneven" is it, and what kind of sander did you rent? It could very easily be that you just need a bigger sander or a more agressive grit on the belt/drum and that that will chow through any unevenness. If any of these warps and uneven areas are basically cosmetic and 1/4" or less in surface height difference you might be able to sand them right out if you do it right. If you find the biggest, baddest drum sander in your town and start with a very aggressive grit of paper. Some of these grits get so rough that it looks like there are little pebbles glued to the paper. You should be able to chow through most surface irregularities with this, also will take all the stain off. It sounds like you were using some wimpy sander and breaking your back using all the wrong stuff on this floor. The only caveats to all of this is that these warps and ridges should not be moving. If these irregularites are associated with relative movement between the boards when you step on them then this needs to be addressed first by re-nailing and/or screwing, etc. Also, if the floor was sanded down many times before there is a chance there isn't enough left to sand out, i.e., tounge and groove floors can only end up having the top 1/3 sanded away before the groove is exposed. Sound like you need to remove much more material than you were, you were only scratching at the surface, you need to really remove about 1/8" of the top layer of wood. This is standard practice for stripping old floors.