Just too sensitive to temps. Figured that out just via autocross - the surface overheats. And if you over-drive them a tiny bit and push (so you get a little slide) then the surface seems to super-heat and they take forever to come back.Falling off too quickly?
That and once you've used a proper track tire (Hoosier R7, etc.) it's tough to want to go back to a 200TW.