First of all, make sure it's not malware.
It does sound like the symptoms of an overheated CPU - but you think it's OK? Can you see what the temps are - tricky, you only want to know when it freezes, when you can't see it, and by the time you can it has cooled down!
Other suspects are memory - although that usually manifests itself as a blue screen or complete and unrecoverable lockup - and disk corruption, but again that tends to have more obvious failure symptoms.
If you can eliminate hardware, then what you have to do is to disable all add-ons and programs that normally run in the background to see if there's an errant process like AV that's hogging all the CPU. I have seen servers brought to their knees by supposedly reputable AV software (McAfee and Kaspersky), so that would be high on my list of suspects. Running two firewalls or two AV scanners can have adverse effects.
But the best way to troubleshoot it is to get a spare hard disk and do a clean install on that, or backup all your data and do a clean install on the existing disk.
If the system is completely unresponsive there's really no alternative to a hard reset. Sometimes with laptops you have to go as far as taking the battery out to make it stop!
SD