Gaming PC showing sudden poor performance

I have recently been experiencing issues with my Gaming rig. I was able to play nearly anything I wanted to without issue until roughly one week ago. Around that time, my system was no longer able to play games that I had previously played.

EG: I was able to play Diablo 3 without an issue previously. Now I cannot break 14 FPS. This performance drop is the same for all games now.

Rig:
8GB (2x4GB) G.SKILL 240-Pin DDR3 SDRAM 1600 (Model F3-1600C11S-8GNT)
NVidia geforce gtx 560 1GB onboard GDDR5 RAM
i5-2500K 3.3GHz
ASRock B85 Anniversary LGA 1150 Intel B85
RAIDMAX RX-735AP 735W ATX 12V v2.3

Nothing is overclocked.

Testing performed using Diablo 3:
I noticed that the CPU temperatures were reading 100C at all times. I was unsure what was causing this, but thought it might be a faulty CPU sensor. I decided to clean out my case. I also removed the CPU fan/heatsink, replaced the thermal compound on both the CPU and heatsink and reseated the fan/heatsink. This was able to resolve the issue with the CPU temperatures.

I ran MSI Afterburner to review the changes with my rigs components during a gaming session. The processor never rises above 30-40% load with roughly 60C max temperature, the GPU only utilizes half of the onboard RAM, the GPU does not exceed 50% utilization, the GPU temperature is usually 10C less than the CPU, and RAM utilization stays at roughly 4GB.

I lowered the settings in Diablo 3 to the lowest values. Still, unable pass 14 FPS.

I used Real Temp to check the CPU again and found that the numbers matched MSI Afterburner. I also checked the bios and found no problem with temperature there.

I started the system with a clean boot to rule out any third party applications.

As I already had the latest certified drivers, I installed the beta driver that was available from NVidia. When this did not work, I tried the five previous drivers to the one I was on one by one. A clean install and reboot were performed every time.

I changed the room that the rig was setup in. I also checked the voltage to the processor. While I am unsure what to expect here, it appeared to match what others were showing.

I have reviewed many threads in an effort to find a solution, but I seem to be missing something.

Images: http://imgur.com/a/w4iJS (Images for this issue.)

I used the Hard disk sentinel and could not find any issued with the HDD or SSD.

I ran a memory diagnostic test and also could not find errors. I also tested the system by testing each stick of ram in the primary ram slot. The system did show a 5-10 frame improvement with both sticks. I am starting to think the issue is with the RAM

edit add: Upon review of the Graphics card, I found that every tool indicated that the clock speed was stuck at 51MHz rather than the 851MHz. The GPU memory speed is also at 135MHz rather than the 1002MHz that is expected. Not sure why the clocks are stuck at the lower speed if the system is idle or under load.

Any suggestions or ideas will be greatly appreciated. Do you think my processor might have been damaged from the heat? Is my power supply a likely culprit? Is my graphics card dying? What additional steps should I look into?

New Update:
I have tried a few things now to resolve the idle cores. I still have not been able to resolve the issue.

I have gone into the Nvidia console and turned the power settings to prefer performance. This solved many other users problems. This does not seem to have had any effect on my situation.

I have uninstalled the drivers for the video card as well as the Nvidia related software using Display Driver Uninstaller. I then manually installed the latest working driver.

I found that the BIOS for the graphics card was one behind. I flashed the BIOS for the card using NVFlash.

At this time, I am thinking that I am running out of options. Is there something in windows that is potentially causing this issue? Is it possible that my video card is finally dying? Could my power supply unit be providing unclean power or something? Could reinstalling Windows potentially resolve the issue I have found?

Again, any assistance will be greatly appreciated.

1. I will double check for malware. I thought this a likely culprit at first as well. My initial searches did not find anything. I will try again.

2. I can assert with much certainty that I utilized the correct drivers for my system. I utilized the GeForce experience option as well as http://www.nvidia.com/Download/Find.aspx to pull the older drivers. In the interest of completeness, I will attempt to install a driver from the UK site you referenced.

3. While I will not argue that the GPU appears to have some issues with the cores fully spinning up, the CPU is not experiencing this issue. This can be seen in the images that I linked to. Specifically, the image labeled D3Screenshot. If you have noticed something I have not, please let me know. but considering that all cores were showing processing time during the execution of Diablo 3, I do not believe any cores are parked. In the interest of completeness, I will review the core settings and let you know.

4. My monitor is indeed plugged into my GPU. I have seen many people make this mistake and am glad to say I am not one of them today.

5. It would seem that your list is a bit out of sorts for 5.

6. As I stated in my updated section, I utilized the Display Driver Uninstaller and will do so again when attempting another install per your recommended site.

7. I have many tools that help me to review the hardware configuration and status of my system. I thank you for pointing out another one for my future use. I will review it and if it seems good I will test it out. I will let you know if I find any different results from that tool.