Computer shuts off and freezes on start up

Hey,

I built a computer about three weeks ago I started having problems with it randomly shutting off. I've never had it shut down while I've been using it but after I've been away from it for awhile I will sometimes come back to find shut down. There doesn't seem to be a certain time interval that this occurs at. Sometimes I'll be gone for hours and it will still be on when I come back and sometimes I'll leave for only 30 mins to come back and find it off.

I've also noticed that sometimes on start-up it will freeze at the "Starting Windows" screen (I'm running windows 7). I don't think it's a problem with over heating cause I can play a game like "Sleeping dogs" for hours at the highest setting and the CPU stays at an acceptable temp (I'm using SpeedFan to check the temp).

Also, if I uncheck the box "automatically restart" in system failure in startup and recovery, instead of powering off the computer will just go to a black screen and have to be powered off. Any advice would be greatly appreciated.

Thanks.

Here are the specs for my computer:
CPU: Intel Core i5-3570K 3.4GHz Quad-Core Processor
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler
Motherboard: ASRock Z77 Extreme4 ATX LGA1155 Motherboard
Memory: G.Skill Ares Series 8GB (2 x 4GB) DDR3-1600 Memory
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive
Storage: Samsung 830 Series 128GB 2.5" Solid State Disk
Video Card: GeForce GTX 670 2GB Video Card
Power Supply: XFX Core Edition PRO550W (P1-550S-XXB9) 550W