Cisco 7960 losing registration

We have multiple Cisco 7960s installed and have a curious problem with one of them. The phone keeps losing its registration. Upon examining the phone’s SIP configuration settings by looking at “Settings/SIP Configuration” we see that item number 10, “Register with Proxy” is set to “no.” We set it to yes, the phone registers, and then about 45 minutes later, the phone is unregistered and the “Register with Proxy” field is set to no again.

We’ve been scratching our heads here and don’t have a clue. Any ideas?