IPv6 high packet loss WNDR3700

I have a Netgear Genie router, using Comcast Xfinity as the provider.  From time to time, it gets into a state with the following symptoms:

1. I get either very high (>90%) or total (100%) IPv6 packet loss to well-known destinations, such as "yahoo.com"

2. Cisco AnyConnect Secure Mobility Client (used on 1 device) has trouble connecting, often waits a while after taking my password and then complains "Authentication timed out"

Going into "Advanced" → "Advanced Setup" → "IPv6" shows that the router does have an IPv6 address "On WAN" and "On LAN", and I can ping both of those just fine. Changing Internet Connection Type to "None", waiting a while, and then back to "DHCP" or "Auto Detect" gets back the same address, no improvement in packet loss.  

Restarting the router generally fixes it, and generally retains the same IPv6 address, but obviously that's disruptive to other people using the network.