I'd do the following:
1. Post your temperatures, underload and idle.
2. When you ran memtest, did you test a single stick at a time? If not, do this.
3. Remove unnecessary hardware from Motherboard.
4. Does your computer freeze when opening Task Manager when not connected the internet?
5. Download the diagnostic tools for your hard drive and test, they pickup issues that a check disk may not.
6. Use a spare hard drive for a new installation of your OS, if possible.
1. Post your temperatures, underload and idle.
2. When you ran memtest, did you test a single stick at a time? If not, do this.
3. Remove unnecessary hardware from Motherboard.
4. Does your computer freeze when opening Task Manager when not connected the internet?
5. Download the diagnostic tools for your hard drive and test, they pickup issues that a check disk may not.
6. Use a spare hard drive for a new installation of your OS, if possible.