Is your disk healthy? That sounds like the behaviour of a dying harddisk (system locks up waiting for disk I/O as the harddisk retries, retries, retries until it finally gives up). Is there nothing in Console.app when this happens?
No, nothing in Console.app. I have run various disk diagnostics tools and they come up with nothing. My best guess is that it is the gfx drivers, but I have not found a way to manually re-install those on OS X.
