Sudden frame rate drop every 30s in DX 11 applications, please help

I've had my pc running fine for 3 years, and been having the GTX 480 for 2 years w/o any problems. 2 months ago, I stopped playing game (but I still use my computer for other things) because I was too busy studying. Yesterday I started playing BF 3 again after 2 months, and I noticed that my fps drops to 5-6 fps for a few seconds every 30s, it feels like I was going through a slow motion state for a few seconds in every 30s or 1 min. The game will work fine for 10 min, then the fps drop will appear. At first I thought it was the patch of BF 3 that messes things up, but later when I ran the Unigene heaven 3.0 benchmark, I noticed the fps drop again. Then I tried all the games I have in my pc, and the fps drop happened only in DX 11 titles, namely Crysis 2, Far Cry 3, BF 3. Other DX 10 or 9 games, like Black Ops 2, Fifa 13, DOOM 3 BFG edition, Shogun 2 totalwar played smoothly. To further confirm the problem, I tried other DX 11 applications, like Stonegiant demo, 3Dmark 11 and as I expected, the FPS drop appeared in all of them (I did all of those test with the factory clock speed, no overclock here)

I tried all the tests I know of to test my overclocked CPU (Prime 95), my RAM (memtest), my HDD and could not find any error. For the video card, I tried rolling back the driver to 2xx ones (was using 310.90 when the problem appears) and couldn't get rid of the FPS drop thing. I even tried wiping my system clean, did a fresh installation of windows and still the FPS drop thing is there. I have never had this problem in my 12 years of building and using computers.

I think that it's my GPU causing the problem, but I don't understand why only DX 11 applications show the issue. Can any one please give me some advice here, as after 36 hrs of "wrestling" with my pc I still can't find a clue of what is happening...