Perhaps I'm blind, but what OS are you running?
Intermittent system freezing is usually some issue with-in the operating system.
1.) Back-up all important data.
2.) Format & Reinstall.
3.) See if Freezing is still an issue.
If you still have intermittent freezing on a fresh install of your OS, then it's most likely a memory issue or a hard drive issue.
You also can consider the power supply. Additionally, how is your power supply getting power? Direct to wall outlet, UPS or Power Strip.