If it's a server then you should be assigning it a static ip address.

Trouble resolved.

I'm not sure where you got your information, but big changes in NTP, or time shift changes between a host and the authoritative time server on a network (Typically the server holding the PDC role) do not make a server renew its IP. Server's, generally speaking, almost always should have a static IP. There are too many reasons to name, but a few are:

1. Resources that users access regularly could become unavailable at an IP change
2. Other dependancies or other servers that depend on this particular machine might become unresponsive if the intended resources constantly changes.
3. Etc.

Now, about your time being off problem. Do this:

Sync with a domain controller

The very first thing I'd do though, is set a static IP on your server.

EDIT

I misunderstood you the first time around about the NTP issue, and what you are meaning is that the DHCP lease expired as the NTP and the server time got off by so much that the server thought it's lease expired. Is that correct? IF so, then it most definitely is an issue with the time sync and my link will help you. Let me know.