Strange Ubuntu DNS replies

I am having a strange issue with two of our Ubuntu boxes. When the user pings an internal server ("bob"), the replies it get alternate between an old domain we used to be on, and the new/current domain.

Example:

PING bob.newdomain.com (172.16.1.12) 56(84) bytes of data. 64 bytes from bob.olddomain.com (172.16.1.12): icmp_req=1 ttl=64 time=0.487 ms 64 bytes from bob.newdomain.com (172.16.1.12): icmp_req=2 ttl=64 time=0.276 ms 64 bytes from bob.olddomain.com (172.16.1.12): icmp_req=3 ttl=64 time=0.222 ms 64 bytes from bob.newdomain.com (172.16.1.12): icmp_req=4 ttl=64 time=0.251 ms 64 bytes from bob.olddomain.com (172.16.1.12): icmp_req=5 ttl=64 time=0.277 ms 

I have checked these files:

/etc/resolv.conf

/etc/hosts

/etc/network/interfaces

the company DNS servers

and can find no trace of olddomain.com anywhere. Where else should I be looking??