...and I can confirm, that rF is sensitive, especially to CPU stability.
Regardless is it 6600, 8800, 4890 or brand new GTX 580. rFactor should work 100% stable for days.
I suggest to check if your CPU/motherboard/memory are stable (use Lynx and MemTest86).
I assume your GPU is not overclocked but if it is, bring it back to stock.
If you can recreate the moment when it hangs, then create a fresh installation of rFactor and check if that still happens on one of the default cars/tracks (just to be sure, it is not mod/track related).