Are you wireless clients set to use the Ubee as a DNS forwarder or are the clients contacting the DNS servers directly?
What are the actual client configured DNS server IPs?
If it's using the default gateway IP as the DNS server IP, then the Ubee is configured as a forwarder and may be crashing randomly. Renewing the DHCP lease my force a restart of that process on the Ubee.--
If it's important, back it up... twice. Even 99.999% availability isn't enough sometimes.