Are you overclocking the CPU at all? This kind of thing is common when overclocking the CPU with too low a voltage.
What kind of RAM do you have? Speed? Timings? Brands?
Mismatched RAM can also cause this and all sorts of strange issues, especially when overclocking, or on a previously overclocked CPU. That chip is a bit old, and it's IMC may not be able to handle the mismatch any more.
You might want to disable any overclocks using by loading BIOS defaults and pull the 4 gigger (if you are running 4GB and 8GB sticks), or if you have three 4GB sticks, pull the one that is in single-channel out, and see if it continues.
If that doesn't fix things, try a different CPU chip (try using the same model), and if that doesn't work, try a different GPU, and if that doesn't work, it might be the motherboard or PSU.
I've never overclocked my cpu so i think that's not the issue here
I have three 4gb sticks. I restored my pc today to see if anything change.If not, i'll try to remove one of my 4gb stick and see how it goes
why 3 4gb sticks? you run async channel - that can cause problems
especially if you push it to limits