Maybe your power supply is on the way out but it could be any number of things from the information you've given.
DO you get any error messages or BSODs? is there anything interesting in event viewer? Have you tested your ram with memtest? are your system temperatures within safe peramaters?
Intel core I7 920
6GB OCZ platinum 1600
Samsung 1TB F1 Spinpoint
Samsung SM2443BW 24"