This has happened to me 3 or 4 times per gaming session and it really disorienting... Basically while your racing, all of the music/sounds stop except for the engine, and that stays at a constant rate. So from audio queues alone it feels as if the game locked up, or that your car is somehow damaged and is stuck at a constant speed. The only way i've found to fix it is to force a rewind. 
 
Anyone else seeing this? Its annoying as hell.

I have had this once, after rewinding form a serious crash, I completely the race but the 'box froze straight after. Im sure they are looking into it.

Yeah, i've actually heard a few little audio glitches in the game.  I don't play with music on, so that isn't it for me.  What does happen is when I downshift, the revs will sort of "hang" at the lower speed for a moment before matching the sound at the appropriate engine speed.  Hope that made sense.