The reason for this is that the game can't handle if the used VRAM goes above a certain value - no matter how much VRAM you have. Therefore you have to decrease the following grapic settings which should reduce the used VRAM and stop the crashing. If not, try lowering other values.

- Multi Sampling: 2x
- Environment Quality: 4
- Refresh Rate: 60Hz

This happended randomly to me after playing for a few hours and it didn't go away anymore - even after a reboot. The solution I found for this is the following:

Verify your game files in Steam. Usually no file should have failed to verify, but the important step is that Steam will start it's installation routine for the game (DirectX, etc.) after verifying. Let it run and after that the stuttering should be gone.

Don't ask me how or why this works, but for me it did.

This is the same reason Wireless Controllers didn't work in AC1 and AC2, but this time it's only a problem in Multiplayer. But the solution is still the same:

Download the fix[community.pcgamingwiki.com] and extract the files into your AC:Brotherhood directory.

After that your Wireless 360/One Controller should be correctly detected and be usable.