I'd take a look at your network card and APMD settings.  If the thing's going 
to sleep, it may not be waking back up properly.  BIOS may conflict with APMD 
or something.  I have one machine here that was not able to recover from sleep 
mode because the 3com 3c905 wouldn't talk to the PCI bus when it came back up.  
I have a laptop that will respond to a weak-to-lost wireless signal by 
appearing to lock up as well - bad enough that I've powered it down rather 
than try to recover. 
 
The other thing to suspect with a lost of service based on time is that 
time=temperature somewhere in the system.  That's likely to be motherboard. 

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