There are quite a few people who experience dropped connections on a 10-15 minute basis with many of these new high end routers. Poster Phuzion at DD-WRT forum had this to say:
No there is not any build that fixes it so don't waste your time. The firewall scripts works fine. The problem is really with half-bridge modems that assign the router a public IP but send DHCP from their private LAN IP which is outside of the WAN subnet. The firewall prevents the lease from being renewed since the replies are coming from a different subnet than the WAN IP is in. After the lease expires, the WAN IP is lost, and the firewall no longer cares where the new DHCP lease comes from.
http://svn.dd-wrt.com/ticket/973
I lucked out and found a tech support agent at Windstream who knew her stuff and she walked me through manually bridging the modem, even though it was supposed to bridge itself if a router was downstream, and the issue is solved.