I had a similar issue where my DNS was being overwritten after every restart with old DNS causing a failed system.
Every time I would update it via the System Admin it would update resolve.conf and it would work fine until a reboot.
I believe I finally tracked it down to being hard coded in:
See this page for details on how to config:
It sounds like you are having a slightly different problem then was described here by the original post. Perhaps a new post would be more appropriate? Have you tried using the documented method of changing your IP via System Admin?