Not Warped Rotors... What Is It?

[Sisu guy]

I have a 2000 Boxster S with factory rotors and pads. Have done several DEs with no issues. This weekend I experienced the followingIn the middle of day #2 the following started to happen:
When the brakes were hot and I was doing heavy breaking from high speeds (example breaking from ~ 115 mph the car felt like I was braking on a "washboard" surface, meaning that the car became a bit unstable and started to "wobble"

This was NOT the ABS, which feels different, I also did not have brake fading and I had pretty much the same brake distance as before, so the brakes generally still worked great. And it only happened after a few laps when the brakes were hot.

When the brakes are cold everything feels normal. I checked runout and have 0.001" or less on each rotor, no cracking or signs of any pad material buildup. Pads have at least 8mm of material left.

Ideas?..