Lagging issues

Iam having framerate issues. Anyone help me? I will clear out the details then
Here is the problem:-
So everything was okay for the last 1.3years.. suddenly from the last month wierd thing happened. My computer is running all okay.. no problems while watching movies, listening to music etc. But when I start playing games, after 25-30 mins frame rate falls for around 2-3 secs and again shoots back to normal. For example if iam playing battlefield 3.. iam getting 50-60 fps and suddenly fps falls to 26-27 and then again shoots back to normal. This happens even when am doing nothing and standing in the game. This happens in EVERY game that I play. What I did from my side is to made a clean install of drivers, formats and went to the extent of buying a new power supply unit. and yes my machine is clean and I also have 4 fans running + I got for myself an aftermarket cooer for my cpu. so it isnt the temperature ..But the problem persisted.
My machine specs:-
AMD FX-4100 3.6Ghz
MSI 760GM-P21(FX) motherboard
Corsair Value Select (4x2) 8GB RAM DDR3
Antec Neo Eco 520W Smps
HIS RADEON 7850 2GB DDR5 Graphics card
1TB WD Blue HDD
250GB Seagate HDD

I should also mention the monitor name and the ups:-
AOC monitor 1366x768 resolution
Intex Protector 725 UPS

please help! thanks
 
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