if the machine is frozen solid at a screen, it stays that way. it never seems to "shut down" or do a hard reboot. but it WILL hard lockup. frozen mouse and all

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That sounds like a CPU lockup to me. ISTR there was a known issue with some of the faster (400MHz?) CPU modules for the Lombard locking up like this. Or it could be overheating - worth checking the heat transfer (pad? paste?)