Hi Eddie,
just to inform the community.
I found the issue I described above is also related to a known bug of Neutron DVR implementation on Juno 2014.2.1
Bug #1389880 “VM loses connectivity on floating ip association w...” : Bugs : neutron
This bug is solved in the release 2014.2.2 of Juno (I have just update it ;-) )
Thanks
Ale