Grue, noted your comments…
Aka, have you checked if its a problem with your ARP… how many network adapters you have in your system.
to check arp issue go to commad prompt and type
arp -a and strike enter
if you see and network adapter ip with some unknown internet ip and 00-00-00-00-00-00 in your physical address you are having the same problem as i do
when you reboot router this entry will not be present in ARP cache
If you have two cards and you disable/unplug the jack in the other LAN card , does your connection keep working?