damn, I was going to suggest a reinstall.
I remember this actually happened to me once, but I don't remember how it went away. It occured after I had imported new cars, so what cars are you running?
For a test, if you have the time, I'd suggest a complete reinstall, apply the 1.13 patch, the no cd crack, and GpXpatch, and absolutely nothing else. Then try and do a race - If it freezes there, we'll know it's a big problem. Else, if it doesn't, it might just be a superficial problem that's easily fixed.
Also, I'd install the newest drivers for your graphics card, and perhaps turn sound hardware acceleration down?
Let us know how it all goes, you have a killer system and should be enjoying Gp3!