I've been struggling for this one now for about 3 weeks.
We have a router that boarders a B2B connection and within the config we NAT our internals to a specific set of addresses.
So, for example (addresses have been changed from production):
ip nat inside source static 192.168.1.22 192.168.2.22
ip nat inside source static 192.168.1.23 192.168.2.23
ip nat inside source static 192.168.1.27 192.168.2.27
ip nat inside source static 192.168.1.28 192.168.2.28
ip nat inside source static 192.168.1.77 192.168.2.77 <--- Problem child
ip nat translation timeout 300
What I am running into is the 192.168.1.77 inside local (aka Real) is a DNS server that forwards requests to another DNS server across our B2B connection. Every-so-often, the router decides to stop translating the address and nobody can access certain hosts. I have to issue 'clear ip nat translation *' to get it working aging.