I would make sure I know if the conns are torn down when I see the "outage" (I don't think that is the case).What I would check is if conns are torn down, why they are torn down. You can check syslogs for that.
I don't think they are torn down, there might be some slowness if the CPU spikes for some time and you cpu stays high.
Then I would try to correlate the "outage" with the event. Packet captures using the capture command for a host that is experiencing the outage would also help you to see what is happening with the packets of a conn that "breaks".
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...