[spodefly]

Is your ISP looking for the MAC address in one of your NICs? My router spoofs the MAC of the primary PCs NIC card, and that MAC is what the ISP keys on. If I try to renew my WAN IP with the MAC set to spoof another NIC besides the one they know about, I get a DHCP server unavailable reply.

I know this scenario doesn't exactly match the problem you are describing, but it might be a variable.

[SteelTrap]

I think Spodefly is on to something there...Sounds plausible to me.

[tacticalogic]

What's curious about this (to me at least) is that he's got hard coded IP addresses, and can't ping the router. This seems to indicate the router is losing it's IP configuration. It's like the ISP is giving a 24 hour lease to the router via DHCP, but the router's internal clock is off by 12 hours (am/pm toggled), so the dhcp server is giving it a 24 hour lease, but the router thinks it's only good for 12 hours, and doesn't seem to be requesting a renewal.