Please set the voltage back to 1.2V, higher is not supported and causes problems.
The 0xEF could have been caused by a few things, like malware, a damaged hard drive, CPU problems and some other things.
Please download and install Malwarebytes free (Uncheck trial version in the installation process)
Please run a threat scan with Malwarebytes, a full scan with your AV, scan your system with Adwcleaner and Kaspersky TTDSKiller.
HDD & SSD TEST
Run SeaTools to check the integrity of your HDD. SeaTools for DOS and Windows - How to Use - Windows 7 Help Forums
Do not run SeaTools on an SSD as the results will be invalid.
Run chkdsk Disk Check - Windows 7 Help Forums
Run Prime95 to stress test your CPU. Prime95 - Stress Test Your CPU - Windows 10 Forums
If you're overclocking anything, please stop it and set everything back to default while troubleshooting
Your CPU temperatures will rise quickly while under this stress test. Keep a keen eye on them and abort the test if overheating occurs.