As an Apple Service tech, intermittent freezing can be one of the hardest problems to diagnose. It could be any number of things, most likely the HDD, though if Tech Tools showed no bad blocks, then that may well be ok. I would take the first step as provided in your manual and perform a re-install of your Operating System and see if that corrects the problem, as it will remove any chance that it is software related.
If the re-install still shows the same issue, then it is hardware related.
Take it to the Apple store and have them thoroughly test it and if need be, stay with them until they witness the fault. A faulty HDD cable, Optical Drive or Logic Board could all cause this issue.