The normal xbox bios gets it's MAC address from the eeprom.
Some modchip biosses are designed to work without the eeprom. In that case, the MAC address is somewhere calculated in the modchip bios itself. They might change it every time to avoid conflicts with other xboxes on the network having the same bios running. If you would store the mac address fixed in the bios, 2 xboxes with the same bios would conflict to each other on the network. If you randomly generate the 2 last numbers, there is only a 1 on 65536 possibility to have troubles with that.
Some softmods use a virtual eeprom which is a file on the harddisk storing the same information as found in the xbox eeprom. This might cause the MAC address to change as well when the MAC address of that file doesn't match the MAC address in the eeprom.
There is a liveinfo beta 3 program for windows that can analyse the contents of the xbox eeprom. It might give you an idea if the bios gets it's MAC address from there.
It might be interesting to see if the original xbox bios also has problems to keep it's MAC fixed. You should try things out a couple of times with the mod disabled.
I asume you guys are all having troubles with the MAC address and not with the actual IP address the xbox receives during bootup. If it gets that one from a DHCP server, it can be different every time.
regards.