Check your logs. something should show up there. And, contra the other - if there is a config problem with hardware then the software contributes or triggers the freeze. So, if you are running anything else on the box without freezes, then we look to the software config.
If you have compiz enabled, disable it. And, exactly what freezes? The box is just sitting there, not running programs on the GUI and it just freezes? Does it freeze running Firefox or some other program?
Not to say that you don't have a hardware problem. A loose card or stick or connection can do all sorts of things. But your post is too vague for analysis.
So - it just sits there and as you walk by or something - you notice it has frozen? Or, you sit in front of it, staring at the screen and it freezes?
Does the GUI remain or does it go black? The mouse cursor is seen or not seen? Two hits or more of ctrl/alt/del or ctrl/alt/bkspc do not trigger a reboot?
Is it on AC or DC when this happens or is it liberal with its freezes?
You may be having power management problems triggered by the Centrino cpu which would be a software difficulty. You may need a kernel optimized for Centrino, if there is one. You may need an earlier kernel or a newer one.
Are you running the proprietary ATI driver?
Nice box, by the way.
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