I had a very similar problem on a machine I built up from new parts but ran Ubuntu rather than Windoze. I had crashing at random intervals and failed to find any specific cause. Over the next couple of years I replaced almost every part eiother by buying new or by exchanging with the equivalent parts from another computer. Nothing made any difference except underclocking reduced the incidence a bit. When I had exchanged everything except the case and the PSU I had enough spares to build a second system. As a last resort I replaced the PSU and the problem went away. I knew the fix was a good un when I reversed the underclocking and started to experiment with overclocking........... I nearly made the machine fast enough to make up for two years of slow/intermitent working and I had a spare machine ready just in case.
Rans6..........................