Hi,
I managed to resolve the issue for our case.
The two servers we're using as our pfSense boxes are Dell PowerEdge R210II servers, each came loaded with 2 on board Gigabit Ethernet ports (one being used as the WAN interface and the other for the LAN interface).
In the first instance I had setup the pfSync to use the LAN interface, which I'm led to believe is a big no no, so I then set up a separate VLAN for the pfSync to use, but as this was still using the physical adaptor shared by the LAN interface, it made no difference.
In the end I bought and fitted an additional PCIe Gigabit Ethernet card in each of the servers, set up a VLAN to use the new physical adaptor (not being used by anything else) and set the pfSync to use the new VLAN and since then I have seen no issues with the sync slowing down or the Backup box becoming unresponsive.
Hope this helps.
Cheers,
Jan