I have had trouble with a pi, basically high usb traffic can cause the ethernet controler to drop out. I have a 20 port usb 2 hub plugged into the pi, and it will run for a while then randomly the ethernet driver starts getting errors and drops out.
This is a known issue with the pi, something to do with voltage drops. In any event i ended up using a spare old pc and throwing centos on it. USB issues gone

Ive tried doing firmware updates on the pi, and there is a kernel boot option for the nic but these make no difference.
I used to get lockup and dropout problems with my pi which I eventually solved by buffering the power through an extra usb hub which has no usb sticks in it.
So I now have a powered Plugable 7 port hub plugged into the pi and the pi takes it's power from a USB port on that.
Then I have several powered D-Link 7 port hubs plugged into the Plugable hub, with all the usb sticks plugged into them.
I believe this prevents any power fluctuations from the D-Link hubs reaching the pi through upstream power connections.
Whatever it does, it seems to work.
Interesting... I might take a look into that at some point. Currently using a spare Blackberry charger (0.7a) for the Pi, which has always worked fine when running them headless.
Watchdog seemed to catch a lockup earlier on today and automatically restart, so as a short term workaround it definitely seems to be worth trying that. (I've set it restart if load is over 70 for 5 mins)
I've also set up a cron job to ping my router and restart the Pi if that fails, and one to check if a cgminer process is running and restart if not... Quite a lot of effort, but it seems to be working OK so far
An alternative is to cut the red wire in the usb cable between the pi and your hub. That too will prevent any power fluctuations from your hubs from reaching your pi. Be sure to leave the other three wires intact.
Il give this a shot tonight. Sure this will work though? Were talking about cutting the red wire (which is the +ve wire) that takes power from the pi's USB ports to the hub, correct? I suppose that's logical really, power can be back syphoned to the RPI through that USB socket so I guess it can also pull it the other way, also, the hub is powered, so you don't need to be sending it another 5V...
Alternatively, il need to look into cron jobs and watchdog services as chadgroover has done, but that's new territory for me. I suspect it is probably power related however as when the mining stops I cannot ping the RPI from my desktop machine. I know the Ethernet is just a USB > Ethernet adaptor so really should have thought about this one before...
Its just so unpredictable! sometimes it can be 5-10 mins, other times its 12+ hrs before it locks up! Im preparing a cable now to test when im back home, fortunately my work has thousands of USB A-B cables.