If you're not equipped to do detailed diagnostic work, one thing you could try is to use System Restore to rollback your system to a date before this problem started happening. Another thing you could try is to do a repair install on Windows, where you take you disc and direct the setup program to "upgrade", which will re-install Windows over itself while keeping all your programs. This is usually the fix when sfc finds problems it can't fix, but you have indicated sfc finds no problems.
The way it's acting is classic hard drive failure actually, but hell, maybe you got some malware that your scanners missed or something. You might try Hitman Pro, that has caught things for me that other programs have missed.