While not giving up on the issue, I have to tell you that whenever I have a problem like this, I try to be as pragmatic as possible. To exclude hardware fault, I usually end up doing a reinstall. It's time consuming, I know, it requires careful planning, because it's really a bad way of loosing irreplaceable data (I mean forgetting to backup everything). And it's not even 100% foolproof, because I can install the same - I don't know - bad combination of drivers or something. But if the problem goes away, no more headache, and the hardware is cleared.
It's easier if you have an empty hard drive around, you just disconnect the current one, connect this, and put a fresh system on it...
That being said, if the Event Viewer is not conclusive, I think it's pretty risk-free to give a chance to Microsoft's System File Checker.
http://support.microsoft.com/kb/929833