Random freezes and BSOD

Hi guys I've been experiencing some problems with my pc since last month. A week before I went to the sea it began to freeze randomly(1 or 2 times per day ,expecially when I was surfing the web or playign games, but sometimes also when I was using word). Back from the holidays the problem seemed to have disappeared but 3 days ago my pc restarted to freeze and I experienced some bsod too this time. Sometimes it gave me a bsod and froze during it ( waited 15 min ). When it freeze the speakers emit a buzz and my mouse and keyboard's lights turn off. I tried to format it but it didn't work. After that I followed your guide and until now I :

* Updated every driver except the bios one becouse there are different revisions of my motherboard and there is a different update for each one : http://it.gigabyte.com/products/page/mb/ga-p55-us3l_10/overview/
* Used memtest (I made 2 tests in a row to be sure) but it didn't found any problem
* About the psu I wasn't sure if it was all ok so here is a screen(SpeedFAN) :
MVnK30R.png

* My temps are all fine nothing above 70
Specs :
CPU:QuadCore Intel Core i5-750, 2915 MHz (22 x 133)
Motherboard: Gigabyte GA-P55-US3L
Ram : Corsair XMS3 CMX4GX3M2A1600C9 DDR3-1600 (800 MHz) x 2
GPU : NVIDIA GeForce 9800 GT (1 GB)
PSU : How can I check it?
 
Your GeForce 9800GT is getting old now. Make sure that the fan is free of dust and it is spinning... The power supply voltages look fine... but the power supply could be weak now
PSU : How can I check it? Open the case. Read the label, and tell us what it says
 
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