We have 3 ASA 5520s on site, one of which I've upgraded to version 8 software and version 6 ASDM. My own client software has upgraded when I connected to this firewall.
However when I use ASDM to connect back to one of our older firewalls there is a consistent whereby ASDM loses contact with the firewall after a couple minutes of inactivity. You cannot restore connectivity in the normal way expect to shut down the app and restart. Its causing issues because if you take too long to input a new set of ACLs or other configuration you lose what you've been working on.
I've searched the bugs but found nothing similar at the moment and I don't want to upgrade all of our firewalls to v8 just so that ASDM will work.