I call faulty or overloaded psu:
PSU : CWT psp-575p2p 405w sounds like a crappy psu to me can't find it on google
What are the temps of cpu and gpu ?
GPU ~ 55 C
CPU ~47 C
(checked them now with firefox, speedfan and WMP playing some music)
Indeed the psu is not the best you can find, but it has lasted for 5 years or so, and I don't recall changing anything on hardware/software that would cause the crashes.