crhys, It'll be the PSU.
I had the same problem, the Cube basically killed the PSU I was using (an old Dell PSU with 18A per rail). Everything ran fine initially, but the Cube started to fail in the same manner you're describing, until it stopped working totally.
Changed the PSU and everything started working again perfectly. Now running off a PSU with dual 25A rails.
Thanks for the info HellDriverUK, I am running the two cubes on a Corsair CX750, probably just on the edge of what's possible with that PSU.
Have heard that the CX750 can run well with 2 psu running 3 cubes (1.5 cubes each), will configure it like this soon.