Help! weird adapter "failure"

I say failure because it is not a real failure!

Server 2003 box - 2 NIC's - internal & external (running exchange)

The external adapter keeps "failing" won't respond to ping etc. However
the adapter still "works" in that traffic still goes out (ie web, SMTP)
It just fails all DNS requests - no PING / Tracert etc.....

Any ideas???

thanks


ERoss