Mine isn't running at all well with my CX750 either.
I've ordered a cheap OCZ 600W and I'll see how it goes - I had great service from really ultra-cheap 500W OCZ when I was running lots of GPUs.
I have over 20 Cubes running on the CX750 in overclock mode with no issues. In fact, they are all running in a 3 Cube, 2 PSU configuration in overclock. I doubt the PSU is to blame.
Well, guess what? It was the CX750.
Cube runs perfectly on the OCZ 600W. Just a few hashes shy of 38GH for the last 8-9 hours. I've also discovered the Cubes work better though Slush's proxy than through BFG's proxy.
The PSU I bought is as follows:
http://www.amazon.co.uk/gp/product/B001GLFD54/ - £54.99
WAY cheaper than the Corsair, and it works.
The CX750 is back to it's normal job of powering my two Blades, 2 Jalapenos, USB hub, fans and various USB miners.