When you say the car steers in more and more, I'm thinking that the rear end is rotating too much, perhaps enough so that it then suddenly snaps loose. In theory, in my head, I could see that happening. You could try to stand up the shocks in the back and see how that reacts. Though I don't know what position you have them in. I could be completely wrong and it could be that the rear is actually too stiff and thus suddenly lets go. Any way you could get someone to shoot a video of a few corners?