I ran into an issue recently with a client where we were seeing a large level of packet loss to their newly installed SSG140 cluster. There were three clients sharing the same 100Mbit Internet circuit and they all connected directly into a pair of Juniper SRX210 routers.
After some investigation and troubleshooting the following KB article from the Juniper website seemed to demonstrate what the problem was: http://kb.juniper.net/InfoCenter/index?page=content&id=KB7435
The virtual MAC address for both firewall clusters public facing interfaces were the same.
Resolution? Rebuild one of the clusters to use a different cluster ID and the MAC address generated for the firewalls is different.