Low Fps... BF4

My PC:

Amd Processor Six-Core (6 cores) 3.3ghz

8gb Ram

Ati Radeon HD 7870 2gb

I play with everything on ultra except the lighting that is in high

I'm playing 70-120 fps almost always, but sometimes, no matter where this area, down to 20 - 40fps...

Please Help. Thanks.