GPU getting under voltaged during gaming causing lags

[Xombied]

Hi, I having a wierd issue for the past year. Mainly during summers (like now) whenever I play games the fps suddenly drops and shoots back again..Last year I was asked to get a new PSU.. SO i went ahead and bought Antec Neo Eco 520C power supply replacing my coolermaster Extreme II 525.. But the lag dint go. It used to occur every now and then. Ok now as the summer waved away things started becoming normal as there was no occurence of lags during the autumn winters and spring.. Now from the last week I was again getting that problem. So I decided to check. I started this game Advanced warfare and tried monitoring using HW monitor. I noticed after 30mins of gaming.. the frame suddenly dropped and the voltages from 1.138V came down to 0.825 (idle) and was flickering between 0.900V and 0.952V...and the lag lasts for around 5-10secs.. as soon as the lag went away fps shoots back to 60 and voltages 1.138v. I checked every temps.. While gaming cpu dont even cross 45degrees and gpu 58-60degrees. What to do?? Is my mobo at fault or my gpu??
PS - Iam doubting my house wiring because its very very old. I use a ups doh but still. 
My specs:-
AMD FX-4100 3.6Ghz cooled with Antec C20 cpu cooler
MSI 760GM-P21(FX) motherboard
HIS RADEON 7850 2GB DDR5
Corsair Value Select 8GB DDR3 
250GB SATA + 1TB SATA Drives
5 120mm fans 

Please help