You might be better off asking for advice on the Apple Forums
It sounds as if you are a fairly competent user, there may be other Apple users on here who can help, but as this is Apple specific behaviour, i.e. manual DNS settings not sticking, there might be some people there with a better idea of what's happening.
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I am all Apple gear. Why do you want to set a manual i.p.address. Using DHCP has always been fine for me - no issues.
Can you not simply overtype in the DNS box ?
EDIT Forgot to say - I am in modem mode with an Airport Extreme Router - are you just using a Hub - what model is it (on the base sticker?
-------------------- John --------------------
I do not work for VM. My services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired 2 WiFi,) SH2 in modem mode with Airport Extreme Router +2 Airport Express's. On VIVID200, Talk Anytime Phone, x2 Mobile SIM only iPhones.