Random Freeze if you have ruled out the OS, meaning you made a clean installed and run in the same problem means you have some hardware trouble.
1st thing try a memory test, most Linux boot disk can run the test now from the CD.
If you pass the Memory test, run a S.M.A.R.T program and take a look at your HD healt.
If you suspect the HD is broken you can look for the manufacturer HD tool and run diagnostic tools.
If none of that solve the problem, I would suspect a bad cable or power supply.
If you have access to spare hardware, just swap it out and see if the problem is better or worst.
You can also try to remove and extra card and replace the video card.
Past this point the CPU or mobo are the problem. You will need replacement or new PC.