What hardware are you using. I have a couple ASA 5505's doing weird stuff like this...the VPN could work for days and then drop and not recover, but the remote site still has Internet connectivity and can even ping the headend, but the VPN just won't recover until you reboot the remote 5505. I can't confirm that it is the same issue as you because I am not on site and can only get in via the VPN....when its up.
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...