I'm not sure if this is the correct place to post this, but I can't seem to figure out if it's a VMware problem or not. I have an Ubuntu 32bit 8.04 Server runninng with VMware Server 1 installed and running Windows Server 2003 Enterprise edition. The network connection is bridged to the physical interface and I have a cable modem hooked up to the interface. The modem is capable of giving out up to 5 IPs. Whenever there is high load on the connection, either in the VM or the host, the gateway of the bridged interface +in the VM "+disappears." I never have any problems with the host, but I am constantly repairing the connection within the Windows VM. Any statically assigned addresses always stay, for example, I have another connection bridged to my LAN and connected to a router and have assigned a static IP and Gateway, but that interface has no problems.
Personally I would check if another DHCP server was on the network or if VMware was dishing some out before the router can hand it out.