The 'warping' is a pad problem. They are getting overheated and creating judder. (Rotors rarely warp - that is just what it feels like)
When it overheats and you engage the brakes it deposits onto the rotor. So as you're braking the pad will contact the rotor but at times it will not grip in the deposit locations giving you a vibration.
Slotted rotors should help w/ this as they keep the pads fresh and the pads will take some abuse without overheating.
Also upgrading to higher temp brake pads can help.
-Nick
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