Old domain controller refuses to go away!

I have a problem demoting my old server 2003 DC and wonder if anyone might have any clues.

Built a new server 2003 DC, same domain, different IP. Introduced to the network, all OK. AD replicated and old dc knows to replicate to/from it, new dc knows to replicate to/from old dc. OK so far. Made new DC the operations master for PDC, RID, Infrastructure, Domain Naming and Schema, also made it a Global Catalogue. Left all for a few days to let things settle. Switched off the old DC and discovered that client PCs think the domain has disappeared. Switched old DC back on and all is OK. Try to demote old DC with DCPromo and it thinks it's the only DC on the domain.

Both DCs can see each other, know about each other, replicate to each other.

I'm lost now, any suggestions that I can try on Monday morning will be gratefully received.

TIA.

 

Are the clients PC obtaining the DHCP ip from the old or the new server?

Can the client machines ping the new DC?

 

The new DC is providing DHCP successfully. Haven't tried pinging it as I assume that if clients can get IPs then all is OK.