another possibility is you have a crappy PSU, or a good one thats dieing, or your harddrive is dieing on you, Youll know your HD is going when it makes werid clicking sounds or hums alot, as a last resort you could reformat that solves alot of problems.
I know a PSU can cause a computer to freeze because my computer has been freezing like urs and i traced it back to my X-connect, Im Still waiting for my new PSU to arrive from newegg but im sure it will stop my crashes.