My PC has recently been restarting under load even at stock. I've listed my specs below. I have had this system for almost 4 years now and it has been stable at 4.7GHz (with Vcore 1.5V) and has been running at 4.3GHz 24/7 (with VCore 1.425). About 3 months ago, I noticed the PC would suddenly restart when watching videos, usually only when I opened like 3+ YouTube videos at once, no BSODs. So I started troubleshooting and this is what I have done so far:

1. PC restarts without any BSOD in Prime95 even at stock. If running Prime95 at stock, it seems to take longer to restart unless its already restarted from a previous failed overclock (?suggesting an overheating issue). Running Small FFTs, Large FFTs or Blend don't seem to make any difference (they all restarted).
2. I've tried two different PSUs (both new)
3. I've run memtest many times with no errors
4. I was running the FDk beta bios (not quite sure when I upgraded that) but I rolled back to FC
5. I've tried removing the CMOS battery with AC power disconnected
6. At stock, HWMonitor shows CPU Temp around 50C and TMPIN2 around 55C when running Prime95, when OC'd with a VCore of 1.425V, CPU Temps get to about 60C and TMPIN2 70C
7. When running Prime95, the VRM heatsinks are uncomfortably warm to touch but wouldn't "burn" skin
8. I even powered down all my hard drives within BIOS and booted into Ubuntu via CD and ran Prime95 on that and it also restarted when OC'd. I couldn't get it to restart at stock but I am thinking I probably just didn't give it enough time (it has taken a while, even in Windows, for Prime95 to crash at stock when the system is cold).
9. I've tried almost every BIOS setting imaginable, including increasing disabling all CPU power features, reducing HT frequency, increasing NB Voltage, CPU PLL Voltage, setting LLCs to every setting,
10. I had a PSU CPU Power cable extension cord for cable management, but I removed that too and plugged the PSU cable straight into the board to ensure the cable wasn't faulty.
11. I cleanly uninstalled and reinstalled all NVIDIA and AMD drivers.
12. I removed Flash and Java
13. I updated my SSD Firmware

After all this, I've come the conclusion that either the board has become defective or there is possibly a VRM overheating issue. However, I think my temps are otherwise fine. I did try putting a fan right over the VRMs during one Prime95 stent, but it still crashed. Its hard to reach the top left heatsink though because my Antec Kuhler 920 is right over top of it.

So far the system is still reasonably stable, even when overclocked, and crashes only when watching videos (if I open more than one at a time) and running Prime95. Surprisingly, it hasn't ever crashed during a game, except for once and it was just when it was loading up to start (not while under significant load). 95% of the crashes generate no BSOD (the system just restarts).

Any advice would be greatly appreciated. Thank you.

System Specifications: AMD FX-6300, Gigabyte GA-990FXA-UD3 rev 3.0, 2x4GB Patriot Black Mamba 9-10-9-27-2T 1866 DDR3, EVGA 560 GTX, Sound Blaster Audigy 2 ZS, Mushkin Chronos 240GB SSD, Seagate Barracuda 3TB HD, Western Digital Black 1TB HD, XFX TS Gold Series 750W PSU, Windows 10 Home x64.