My computer started to freeze at irregular times for 3 weeks now.

What happens

• My computer freezes, the video stops. (No graphic glitches, it just stops)
• Sound keeps playing up to some time (Usually 10-30 seconds) then stops playing.
• Sometimes, randomly, the screen on my G-15 keyboard flickers and I see caracters not at the right places. Usually happens for about 1-2 seconds and a bit before my computer freezes.
• I have to keep the power button pressed for 4 seconds to shut my computer down.
• I still hear my hard drives and fans working.
• Sometimes it works with no problems for a full day, some other times it just keeps freezing each time I restart my computer and I have to leave it for the rest of the day.
• Sometimes my mouse freezes for a fraction of a second (Like 0.01 to 0.2 seconds) quite randomly, usually before it freezes.
• No errors spotted by the "Action center" unlike when I had problems with my last video card on this system (Driver errors).
• My G-15 LCD screen also freezes.

What I did so far

• I have had similar problems in the past and I had changed my hard drive (It was faulty), so I tested my software RAID-0 array and it was faulty so I changed it. (I reinstalled Windows 7 with this part). I also tested with unplugging my secondary hard drive.
• My CPU was running at about 100 degree Celsius, I removed the dust between the fans and the heatsink and it's now between 50-60.
• I ran a CPU stress-test and it didn't freeze during the tests (using Prime95 on all cores)
• Ran a memory test (using memtest86+) for a single pass and there were no errors.
• Ran a GPU stress test with ati-tools and furmark and it didn't freeze during the tests. (No artefacts either)
• I had troubles with my graphic card when I got it, but I think that it got fixed with a driver update.
• I checked the voltages in my BIOS setup and they all seemed ok (±0.2 I think).
• I have ran on the computer without problems with Fedora 15 on an external hard drive (Appart that it couldn't load Gnome 3 and was reverting to Gnome 2, didn't want to install drivers since I use it on multiple computers) I used it to backup my files from the raid array to my 1TB hard drive for the reinstallation of Windows. (So the crashes only happenned on Windows) [The external hard drive is plugged directly on a SATA port]
• I contacted EVGA (My graphic card vendor) and pointed them on this question, I'm looking for an answer.
• Ran sensors on Fedora 15 and got this output: http://pastebin.com/0BHJnAvu

When it happens

• When I play video games (Mostly)
• When I play flash games (Second most)
• When I'm looking at my desktop background (It rarely happens when I have a window open, but it does, sometimes)

Specs

• Windows Seven x64 Home Premium
• Motherboard: M2N-SLI Deluxe
• CPU: AMD Phenom 9950 x2 @ 2.6GHz
• Memory: Kingston 4x2GB Dual Channel (Pretty basic memory sticks)
• Hard drives: Was 2x250GB (Western digital caviar) in raid-0 + 1TB (WD caviar black), I replaced the raid array with a 750GB (WD caviar black) [Yes I removed the array from the raid configurations]
• 750W Power supply
• No overcloking. Ever.
• There have been some power-downs like 4-5 weeks ago, but the problem didn't start immediately after. (I wasn't home, so my computer got shut-down)

My current to-try list

• Change the thermal paste on my CPU.
• Change my graphic card with a temporary one and stress the computer.
• Change my power supply.

In this situation, how can I successfully pin-point the current hardware problem? (If it's a hardware problem) Because I don't really have the budget to just forget and replace everything. I also don't really have hardware to test-replace current hardware.