DHCP abnormality

I had a situation where a DHCP server went off line, and clients began to
drop their leases. Some lost the lease immediately, others within the next
60 minutes until the DHCP server came back on line. The shortest lease time
on any of the DHCP scopes served by this server was 3 days. By my
understanding this should not happen.

I saw another similar situation back in 2000 on an NT 4.0 DHCP server, where
when it went off-line, all clients immediately lost their leases.

The following further describes my current situation:
Windows 2000 Domain Controller - only DHCP server for this given site. I
ran DHCPEXIM.exe to export and disable all the scopes on this server. The
server was then demoted, rebooted, removed from the domain and brought
offline. It was then rebuilt with with 2003 server, re-promoted and netsh
was used to import the DHCP scopes back onto the server.
At the point roughly when the machine was rebooting after demotion, I had
reports that a few clients had lost their DHCP leases. As the rebuild
process progressed, more reports came in. I saw one of these first hand on a
computer in an office right outside of the server room. The machine was not
rebooted, NIC was not disconnected, it just suddenly was using an APIPA
address instead of its lease.

Any idea what the cause of this might be????

Thanks in advance!