Just to add that it's not just ATI - I'm having the same problem on an nVidia GeForce GTX 560Ti which I've just added to my machine. Oddly, the previous card I used was a *lot* less powerful and ran the game great through the Alpha test right up to when I took a break this time last year.

I've noticed that it occurs when sprinting through high-density areas on the way to the next without necessarily waiting for all the models to load, as if the game was trying to play catch up, then just forgot what it was doing and didn't think displaying 0 models on screen was an issue.

Logging to the character selection screen and back in fixes the issue, but is hardly an optimal solution.