We upgraded LNS and core routers.
Sounds like IPv6 suffered for a minute or so, which is odd.
But what worked was the LNS changed - we drop PPP and routers reconnect. We even had one customer who was playing WoW. The drop was between 1 and 3 seconds according to the log and his game was not affected. He only noticed because our graphs go purple (fixed over night).
Pretty impressive IMHO.