Hello!
It seems to be the tulip driver,
Apparently, it corrupts IPv6 packets. I have seen this bug,
it was cured by tuning pci settings in bios setup to be more conservative.
The most intersting thing was that only ICMP IPv6 were corrupted. :-)
attaching them, the tcpdump looked perfectly normal to me.
I do not think so, packets should have wrong checksum.
Alexey