This is on every game too, i can run BF3 just fine, about 45 fps, but for 5 minutes i get about 15 frames, then it goes back to 45. People say to redo the thermal paste, which I think would do something, because I used the pre-applied thermal paste, but I want to make sure it works, so I need advice, should it work?
Extra notes: I have the graphic options set on Extreme Mode, i have high performance power setting, i have all the drivers up to date.
There's no evidence it's an overheating issue and you can investigate that easily but I highly doubt it. It's not impossible though but just find the temperature.
It might simply be that the area you are in is very demanding or there's a strange glitch forcing you to 2D mode.
Is it 15FPS in the same area of the game?
Try going to an area that you get 45FPS and just sit there doing NOTHING and watch the frame rate. It's not a perfect test in Multiplayer but it shouldn't suddenly dip down to 15FPS unless there is a heating or 2D glitch.
Try running other games with VSYNC OFF (off to uncap frame rate so run at max temp) and see how they do or even a benchmark utility and see if you keep getting the same score every time.