I have had my computer for about 6 months now and I'm starting to question this power supply. I am running an 8350 and a 7870, neither of which are capable of any substantial overclocks. My 8350 runs at 4.4Ghz on 1.3875v but CPU-Z reads 1.2875v on idle and it drops to almost 1.2v when running p95. In addition, it downclocks to 1400Mhz when it is running at 100% for over a minute, with the temps in core temp only reading 47C.
My 7870 maxes out at 1150 on the core and 1350 on the memory (1000-1200 stock) with a pretty conservative voltage increase (1219-1231). MSI Afterburner frequently displays voltage spikes in both directions about once every 30 seconds or so while playing a game. (With my custom fan curve my GPU rarely gets any hotter than ~63C on full load)
Here is a picture I took a while back of the voltage spikes in Afterburner: http://imgur.com/vE4xG2y
Please excuse the picture quality, I used my phone so I could send it to a friend, and I didn't feel like starting up a game just for the sake of snapping a pic of my voltage.
Corsair tends to have pretty solid PSUs, I'm a novice on voltages and what's normal, but I can tell you for sure that you are well within the wattage capabilities of that PSU... Other than it just being old and tired (not likely at 6 months), or perhaps a lemon, it shouldn't be an issue.
You can probably RMA it if you determine it to be the issue, Corsair seems to be pretty responsive.
They do tend to, but from everything I've heard, the builder (CX) series are an exception.
What motherboard are you using?
It's definitely not wattage that's the issue here. It's probably something more along the lines of amps.
This sounds like an overheating issue. Either your CPU, NB, or VRMs are overheating and throttling your processor probably. Could you tell us what motherboard you have? Have you tried stock settings?
Okay, let me try to explain this as well as I can.
It isn't wattage, and no matter what PSU it is, at 750W, it isn't amps. Wattage is not an abstract, unrelated variable in electronics, independent of the amperage. Wattage, in fact, is I*V, which is one of the many, many ways of expressing current * voltage. Since your components are relying mostly on the +12V rail, and in most cases, you have the +5V and +3.3V rails being derived from said +12V rail, which, iirc, is the case with the CX series, then mathematically speaking, if Corsair hasn't lied, and the PSU puts out 750W (probably closer to 730, actually), then the PSU has to deliver 62.5 Amps (P / V = I). Seeing as it probably isn't a true 750W PSU, it will be around 60 to 64 Amps, unless Corsair really lied about the ratings.
This sounds like a driver problem, not a PSU problem. Ensure both your graphic drivers and Windows are up to date, and see if the problem persists.