My system is similar to yours. The majority of the time my computer hard locks like that is when I have a GPU related issue. Specifically, it's when I overclock my card too high and run certain applications. For example, I can pass benchmark and stress tests without any problems, but when I play BFBC2 or BF3, I can experience that hard lockup. Reducing the overclock solves the issue.
There are a couple of ways to help you determine what may be causing the issues in your system. First, after a lockup, go to Control Panel -> Administrative Tools -> Event Viewer. Under Event Type, look at any Critical, Error, or Warning events that just occurred. If you double-click the event, it will show you more information related to the error. In my case, I will see a Display error referencing the nvlddmkm driver, indicating that the NVIDIA driver crashed.
Another way is to check your crash dump files. Depending on your Windows configuration, they may be saved in either C:\\Windows\\Minidump or C:\\Windows\\LiveKernelReports\\WATCHDOG. You can use a program such as NirSoft's BlueScreenView to read these dump files. The possible drivers that caused each crash will be highlighted in pink. If you see nvlddmkm.sys or another driver starting with "nv", an NVIDIA driver was related to the crash.
One thing you can try, if you haven't already, is to remove your card from the computer, give it a good cleaning, and reseat it.
The only other thing I can think of is that it could be a power issue (PSU). I'm not really sure how to test that since I've never had to do it.
2. I'm not sure if you've read my issue post clearly but as it is a system fault/hard freeze, there are no BSOD or system dump files. Nor is there any events in the Event Viewer console besides the Kernel event stating that an "unscheduled system shutdown/restart/power down has occured" due to the need of me having to use the reset/power button since the system absolutely does not respond to any keystrokes or mouse click inputs.
3. I've been opening and cleaning my system/case for over these past 5 months hoping that it was only a cleaniness issue but this is not the case. All components that can be removed easily (minus the CPU/CPU cooler) have been cleaned repeatedly and correctly cleaned without damaging any connecting parts and reseated.
4. If it was a PSU issue, I've searched around for PSU related issues and most issues with PSU would involve the system being unable to boot up at all due to insufficient power provision or the system would appear to reboot on it's own due to unstable power provision by the PSU. Apart from that I do not recall any other PSU related issues that relate with my current issue.
Running the SMART test again using the one given by SpeedFan now, maybe this'll end this wild goose chase.