My suggestion is to start from basics, and try to eliminate the problem.
Start unhooking anything and everything except the RAM, processor, and video card.
I saw you have a 550W psu. Is it a generic? or namebrand like Antec, Enermax, or Thermaltake? If it's generic, or even if it's a namebrand, could be a problem with the psu starting to go bad on you. Try a different one, if you have one.
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