Another thing you can try to get some idea if this is the machine going off into the deep, or a hardware fault, is to bring up the task manager before the freeze and set up for "always on top" mode. If the fault is software related, the t/m will show which task is consuming all the resources and also the state of memory, swap space etc. The clue may also be in the h/d activity light, which may be thrashig due to not enough memory, or even a disk fault, giving the impression of a lockup. What happens if you just leave it for a few tens of minutes?. Does the h/d light eventually go out, for example. If you put your ear near the drive, does it sound active, ticking noise, whatever ?.
Do try reseating all the hardware connectors and parts though, as suggested in my previous post. Ime, this is the most common cause of unrecoverable lockup where the software hasn't itself crashed....
Regards,
Chris