Brake in a straight line, ONLY, Get gently back on the throttle coming around the turn, and floor it when you're going straight again. It sounds slow, but it's a d*** sight faster than spinning out just past the apex of the turn.
What kind of HP are you running in it? I've found it is a little unstable under braking but with stock HP or a mild upgrade, getting back into the throttle usually transfers enough weight to the rear that you stay in the driver's seat.
Did you by any chance turn the rear downforce all the way down for a Test Course run, then forget to turn it back up? Because it is pretty slippery with the downforce minimized.
With about 576 hp I was turning 1':49.mid" at Grand Valley using T2s and a pretty default suspension setup:
18.0/16.2 springs
76/76 ride height
7s all round on the dampers
0.80/1.20 downforce
Thank you, sir.
I hope my suggestions help.
If they don't my next question would be how do you have it set up.
If you have ASC off turn it up to 2.
Set LSD to 10/30/10. Increase decel to 15 if the car is still getting squirrely.
jus like gil said brake in a straight line but if you want good responsive braking and other stuf go to www.gtvault.com they have a whole bunch of tuning setting and other stuf