My Uno with the enc28j60 shield is more stable, but still not THAT stable. Will go a week, sometimes two without crashing. Now it just locks up for no apparent reason. Haven't had a chance to troubleshoot it. interestingly, it always locks up now at a time when the 'receive' (green) LED is lit, so something about receiving a message over the radio is locking it up...
I tried using the wiznet shield, but so far I've gotten two bad ones (even the built-in Arduino samples don't work), so I don't put much stock in them any more.
I have been planning to swap out my enc28j60 shield for just the little detached module and see if that helps. Just amazes me as much as folks have banged on this stuff for all this time, that it's still unstable, but I guess each one is different, knock-offs, etc.