NBNS entries

HI,

We recently replaced 2 old windows 2003 dc'S with windows 2008r2 dc'S. All is working ok except on odd occasions when query the AD i get errors such as "Server not operational" or "Domain does not exist" Everything looks correct on DNS and there are no entries for the old servers so i am looking at alternatives. If i run wireshark i get entries such as

192.168.180.132 192.168.180.255 NBNS 92 Name query NB XXXXXXX

192.168.180.132 is the ip of a standard workstation 192.168.180.255 is an ip that has no harware associated with it XXXXXX is the server name for one of the old servers i removed

I am fairly new to all this. can anyone shed any light on it for me???