OK so the components you need to know about in this scenario are:
Intel Core 2 Quad Q6600 G0
Corsair H60 Hydro Series CPU Cooler
Gigabyte EP45-UD3R Motherboard
G-Skill PC2-6400 800MHz RAM
Rosewill Value 2 650W PSU
Now I know about the PSU but I wanted to make absolutely certain that it could generally be the issue. But let me explain the situation first.
This PC has been running at a 3.6GHz OC for about 6 months now without any issues with the exception of recently. I decided that since it's been running stable for about 6 months that I should probably be able to run a stress test on it using PRIME95 and have little to no issues. Well, it would seem that way. So before running PRIME95 my CPU usually idled at 40C -32C (that's the temp difference between core 0 and core 3.) which has given me no issues, and for what I use the PC for, I've had no issues with temps usually capping out around 54C under my regular use load which is roughly between 40%-80% usage.
Well, I decided to stress it. Using small FFTs it would prime for about 5 minutes. Tempuratures would climb from idle to about 67C under this setting and sit there for the remainder of the stress. But noticably, at the 4th small FFT per core, the computer would shut down, without warning or error message. (BSoD) Simply shut down, and power itself back on as if nothing happened, and boot into Windows normally without any pre-Windows messages stating the computer did not shut down properly. BIOS settings would remain the same.
Now it does this, consistantly, on the 4th small FFT at 8KB. Currently, the CPU is sitting at 1.425v before I tried upping it, ending at 1.500v and deciding to come here for advice. I ran the CPU at stock settings of 2.4GHz and stock voltage and ran PRIME95 for 16 hours without incident. Just to make sure the CPU wasn't damaged. I then ran Memtest for 19 passes before I decided it wasn't the RAM.
Maybe related, I had issues when OCing my Sapphire Radeon HD 7850 where it would run fine at normal temperatures and suddenly the screen would go multi-coloured and I'd have to hard reset the PC to get it going again, where it would continue to function normally until I ran games, where it would repeat, with different kinds of screens (sometimes black, sometimes multi-coloured lines, sometimes a solid green, but nonetheless the computer locked up with it everytime) until I removed the OC, where it would function without error every single time, even when running Furmark's burn-in test.
Generally, all signs point toward the PSU, or so every single Google search would say. I'm planning on replacing it this coming Friday. Not sure which type yet. I know 600w would be sufficient however I like being over than under, so 700w would be what I'm interested in, as well as fully modular, so if you have any suggestions for a PSU that is good quality while price friendly, please let me know. Pretend money isn't an issue (because it isn't) but still, I'm generally pretty cheap so try and keep it in the $150 price range. If it had a pretty white LED fan, even better. I'm a sucker for aesthetics. But most importantly, it has to be FULLY modular, and preferably from Newegg Canada.
Overall though, please let me know what you might think it is. I'm open to anything with the exception of the standard "I don't know what I'm talking about but your mainboard is failing.... derp!" I'm more than interested to see the community's response as I've come to trust some of the information shared on this site and I'm absolutely stuck. Under regular use, my PC is perfectly fine. Absolutely flawless! However, I want 100% (if not, damn close) stability with this OC. So please, help me out.