One other thing to note - if possible, FFCore should have a static IP address. If you use DHCP and the lease expires then you will see problems with connectivity to the DFEs. You would recover from those via reboot. That may also by why connectivity is suddenly restored after install.
In most cases DHCP is not a problem since leases for a server that's always available on the network would not expire. However, it is recommended that you use a static IP address to be on the safe side.