10.4.11 DHCP Issues

Hi All, X Serve G5 10.4.11, 10.4.11 & 10.4.10 clients (about 150). D-Link and Dell gigabit swithches, Zywall 35 router. I have issues with DHCP and TCP/IP configuration on my network. Until few days ago, there were no issues. I have now some Macs that to do not get IP address asigned by DHCP server (only running on X Serve). This is intermitment. Sometimes they are configured correctly, sometimes they just hang with self assigned IP. In this situation, even configuring these machines manualy with fixed TCP/IP settings doesnt help. Machines can't contact router or G5 server. Sometimes they can contact local LAN machines, but are not able to contact anything through the router. On the DHCP server, machines are often listed with IP already assigned, in the client list, yet in the log I can see they are still sending requests. Other machines are discoverd, offered IP, but never ACK the offer, hence not getting IP. Static configuration using MAC address works, but it is not option in our environment. Swithches were rebooted, but the issue is not localized to one switch, it is accross entire network segment. Any ideas please? Thanks, Stepan