Trouble Connecting Two Macs Via Firewire

Hi,

At work we use an iMac and a Macbook pro, both late 2012 models.

We often have a need to share very large files, and found that a firewire Ad-Hoc network allowed us to do this no problem, its worked fine for around 12 months now and all of a sudden stopped.

- The Macs used to connect perfectly via firewire, each having access to the others files / folders with no limitations.

- All of a sudden they only connect to eachother using a W-Fi network they both run on. This makes moving files unbearable. (Firewire would take a minute)

- I've tested the Firewire connection physically works by starting the Macbook Pro in target disk mode and accessing files from the iMac no problem...

- ... but start them both in normal mode and it defaults back to Wi-Fi again, even though FireWire shows as connected!

- Firewire is connected with a green light and set to DHCP.

- The only change we can think of is an update to OS X 10.8.3?

Any help would be great as it's slowing down the rate we can work, and if anyone knows a different method of sharing large files between two Macs please share..

Thank you!