Why does my pc restart for no reason?

My pc has recently been experiencing problems. First it started when I was playing video games, the pc would just restart during gameplay, then it would do it when i was converting videos for psp, and now it will restart if I have to many browsers open. At one point my pc was unable to restart to windows so I had... show more My pc has recently been experiencing problems. First it started when I was playing video games, the pc would just restart during gameplay, then it would do it when i was converting videos for psp, and now it will restart if I have to many browsers open. At one point my pc was unable to restart to windows so I had to reformat(cause I got the blue screen of death). I changed the memory, and that didn't change anything, even changed the slots they were in. I bought a new video card, and tthat didn't change anything. All that I can think of is the mobo. Any thoughts?
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