If I shut-down my pc it sometimes restarts by it's self. I doesn't always do that, but it got quit often in the last week.
I always had to disconnect it from the grid, to make sure that it won't restart itself. I don't think that's to great for the CPU/GPU if I just disconnect it while it's running. I have a 700W Flowerpower PSU that is 80+ Gold and it is not overloaded.
If someone knows what is wrong with it please help.
Judging from the parts in your profile, that should be more than enough for power. The 770 requires a 600 watt PSU, so you are good there. To me, it sounds like the motherboard or PSU is going bad.