Hi,
When the system is frozen, does the capslock light flash?
hadn't payed attention to this, will do so next time.
The lack of error messages in the log may mean the system is too frozen to write to disk. Can you make it freeze from the text console (Ctrl+Alt+F1), and if so, are there any error messages there? If not, you can also try https://wiki.ubuntu.com/Kernel/Netconsole.
Well, I couldn't get the system to freeze while in text mode and haven't had time to set up the net console method. Will do so after my holidays.
Cheers -- Torsten