PSU dying or something else?

Hi,
I've posted this problem initially under graphics cards, as I believed initially that was the problem, but now I am almost certain it is not, and need some help if possible. Two days ago, while gaming, I started getting screen artifacts and lockups resulting in a restart. This happened while playing games, using firefox, even typing in word. It also began starting right as windows was loading startup at desktop.

I immediately checked my video card, cpu and memory with various applications, cleaned out my case (resulting in a very dramatic temperature drop in my cpu), put a fan by my open case to improve airflow, scanned for viruses/malware/spyware, and reinstalled a bunch of drivers.

Each effort seemed to help at first, but eventually, either in a few hours or the next day, the artifacting and crashes would occur once more. Now, two days later, there is no more screen artifacting, but the crashes remain. My screen simply freezes up and restarts.
I have also begun experiencing boot difficulties, after a restart when I try to power up, it simply hangs and my monitor stays off. The fans spin and the lights are on, but nothing is happening.

I was not overclocking anything at the time this started happening. I am confounded as to what the problem is. Sometimes my pc won't boot, sometimes it crashes while in windows, but doing nothing, and yet I have also been able to run gpu and cpu intensive apps for hours a few times without conflict. I've run realtemp's cpu loading program and another, stress testing it, and it does not go above 50 degrees. When I play crysis or gta4, my video card does not rise above 78 degrees. Is this the psu failing? The motherboard? Or perhaps the hard drive? Can anyone help me?

My system specs are:
Core i-7 stock fan, stock speeds.
Corsair 1333 mhz ddr 8-8-8-20, stock voltage.
Rampage 2 extreme motherboard.
Sapphire radeon 4870x2 stock speeds and fan (its one card, not crossfired).
Seagate barracuda 500gb HD.
Windows 7, 64bit.
OCZ Z series, 1000w, 83 amps @ 12v.
This system is barely over 2 years old.