Macs dropping off Active Directory Network

[it10]

Small number of Intel Macs running Leopard (10.5)

AD is Windows 2003 R2

Since last week, Macs have started dropping off the AD network

. DHCP is working - they all pick up addresses
. DNS is OK
. Time is OK (synching with AD time server)

Attempt unbind/bind. Unbind works, bind doesn't

Strange thing is, nothing has changed (that I'm aware of)..

. No new MS patches installed
. No recent OSX updates

Nothing in DC Event Logs to suggest any problems

Any suggestions as to where I might look?

Thanks