Does BF drop out or slow up at that point too? If so, I'd go with temps causing something to throttle back. Could you monitor temps and see how high they go, and let us know what case you have.
No. BF3 does not drop in fps at all. my CPU temps maxes out at 57-60'Celcius, and GPU temps maxes out at around 60'C as well.
I have a Coolermaster Haf 922 Case, with a corsair H80 liquid cooling kit.