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Something (Google?) keeps changing the third DNS server to google: 126.96.36.199. I do not want to use google DNS servers, 188.8.131.52, 184.108.40.206, etc.
I do have the System Preferences-->Network parameter pad locked, but the third DNS entry gets changed anyway.
When you say "Mac" do you mean OSX operating system?
One thing I would check are the WAN settings on the router and see if you have static DNS there. Next thing I would check is if a static 192.168.x.x IP address makes the DNS stop changing? Besides that, I also found this Apple forum post with gives some suggestions too.