Hey.
You figured out what the issue was ? I am currently setting up cisco 7940/7941 phones connected through a Dell powerconnect 6224p switch, and i am experience the same problem you had.The phone registers fine on powerup, but in a while it looses its configuration and goes back to register again, and stays there until i powercycle it.
The LLDP information on the Powerconnect also dont include any information, seemingly it wont speak with the phone, or vice versa. I have considered everything but cannot find the problem. Would be absolutely killer if you could point out what did the trick.
Phone is using LLDP to get vlans/dhcp etc, not much static configuration on the phone itself