Does your WAN MAC address change after reboot? I'm wondering if it's related to the same issue I'm having. See my post here.
Basically, with DD-WRT on my router, on reboot all interfaces on the router have the same MAC address, including WAN. After clicking "apply settings" on any configuration page, the WAN MAC address changes to what it was on boot +1.
If you have an ISP that only hands out a single IP address to a single MAC (sometime rebooting your modem, then your router, will allow you get a new IP address on a different MAC address, sometimes you need to call your ISP to report your new MAC address -- depends on the ISP.) the MAC address changing will cause you to lose your WAN IP.