My recent ZigBee nightmare was a little different. The Gateways would fall offline then needed to be rebooted to come back to life. I tried everything and think I finally figured out that it does have something to do with Network traffic. Once I put all of the Zigs on a different subnet, they have seen to be more stable (about 3 weeks now, knock on wood). I haven't been able to isolate the exact network gremlin on the network, but a couple of Zigs would always fall offline after the kids had a long night of Xbox360 escapades.
I've had masters lock up and fall offline in the past from some sort of Cisco traffic. My short term fix was to put the masters behind a router and only allow necessary traffic. I beleive this issue has since been addressed in firmware.
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Just some food for thought.