Having compared yours with mine, the only big difference I noticed is that your's ECHU is [16 0 0 0] where mine is [0 0 0 0]. Tried searching what that means, but couldn't find anything, maybe some other member can clarify. Also have you tried running it with a voltage offset of 0? Just curious what it would give then.
I ran it at -2. If i set it at 0, it will restart after 5-10 mins
Look at the System Log tab, that might provide some clues. See what's happening around the time the hashrate is dropping to zero. Maybe there is something happening with dhcpd, etc.
The 741 has a watchdog running that will reboot the system ~5 minutes if it hasn't sent work or if the ASIC stops responding or if the cgminer API isn't accessible to the local host. If the machine reboots on voltage of 0, it's likely that the ASIC is dropping a board (or a row of chips) prompting the watchdog to reboot. Swap your PSU and see if that helps.
That said, have you made any changes to the API addresses in the cgminer config tab? That one drove me crazy for a day... 127.0.0.1 must have access to the API, default is "W:", but I'm not sure if it needs write access.
Good luck!