Our web server in the past day has dropped two static IP addresses. When they are dropped, they are still listed in the IP Properties, but if you perform a "cmd" -> "route PRINT" they are not. Removing the IP from the IP properties, and adding them back corrects this, but any idea on how they are dropping? There are no helpful entries in the system logs.
I encountered a very similar scenario on a 2003 server we have set up. I ended up having to replace the NIC to correct the issue. I would not see any entries in the system events, the ip address would just switch from static to dhcp.
Swapping out the NIC solved the issue for me.