I have a problem with my cube
I have taken it all apart and tightened the heatsink screws and re seated the boards
It runs off a Corsair cx750 M
somewhere in this forum is a reference to certain corsair psu's having a power saving mode that cause's issues similar to this. I believe it was an AX model but may include the CS model as well. I know for a fact the GS800 works fine.
I use the AX860i's to power two cubes each; no issues. Someone stated it was a GS (Gaming Series) that did it; sounds now from your experiences that it could just be a bad Corsair power supply that's failing (the GS).