Time for a Mea Culpa which caught me out badly today.
ALWAYS Check that your [NET ADDRESS] field on the PORTS tab of the Server Record is set to be something other than an IP address.
This happens to be something that just slips my mind and if you do forget and put an IP address in there. It can reach up and bite you on the bum like it did to me today when I spent a whole morning wondering why the F**K a server was communicating to an old IP address which wasn't assigned by the DNS, wasn't in a HOST file, wasn't on the connection documents. The server just wouldn't be told.
Then I remembered the Server doc NET ADDRESS fields.. and yes there to my intense embarrassment was an dotted decimal address where a host name should be. It had been probably been there for years and it was only now when the physical hardware was swapped out that everything stopped working.
So let my blushes be a warning... don't forget your NET ADDRESS fields!