Cisco Routers and CCR IPv6 sub-delegation

Folks, I am seeing some strange behavior that appears to be related to DAD on popular Cisco routers and sub-delegated IPv6 prefixes. When the issue appears I see intermittent IPv6 connectivity from behind my internal router. This is what I had done to stable the router:

I found that if I enabled v6 DHCP on the NetGear Cable modem via the LAN IPv6 Setup Page and change the "Valid Lifetime" to 3600 all was good..