What settings are you trying to play BF3 at? It should work flawlessly at high and at ~40 fps with ultra without AA.
When i play bf3 by itself its fine btw i play with fxaa injector and low post aa and low settings
it only happens when my cpu starts to max out. its weird. when i start the stream with xsplit my fps goes down alot then comes back up but not very high around 50 which is unplayable for me