First assumption is a bad SSD (primary) -test the heat-
Second assumption is a bad HDD (secondary) -test the heat and ticking-
Third assumption is a software program causing a memory leak, especially software that is set to monitor activity all day like back-up, stock-trades etc . . .
Forth assumption is bad Ram, CPU and/or MB
Oh, there might be one other thing. Check for media in your optical\thumb drive(s) as some programs (esp games) cause a system to hang just as you describe. I know it happened to me up to W7. I rarely have media in my optical/thumb drives nowadays, so I don't know about W8.x
Sorry, all I got are assumptions atm
Corsair 500R Case, H110 Hydro, 1200AX PSU, Asus Maximus Hero VI MB, Intel 4770K CPU, Gigabyte GPU GV-N98TWF3OC-6GD, G-Skill Trident 2400MHz 32GB, Crucial M500 960GB SSD, Seagate 6TB HDD x2, Creative SBZ to Denon AVR-4311ci - Infinity Primus 5.1 w/Klipsch Sub XW-300d, HP ZR30w 30" S-IPS LCD, W10 64bit