New PSU
I went into town today and bought a 70e Antec Neo Eco 620C 620W power supply for my PC. Not too shabby considering a 600W Corsair is about that online with tax and postage.
So far it seems to be keeping it stable under fairly high load (including another build of CM7 n85+) for 4 hours, so hopefully that was the cause of the crashes. I noticed this new one has 48A on the 12V rail whereas my old 550W one had 20A or something stupidly low.