Another person here with an Aptus 75, and Firewire issues.
I've had problems since I first got the back. Shorlty after buying the back, I did a live-view shoot, to a PC laptop via a powered Firewire repeater, the board on the Aptus fried. I'm pretty sure it had something to do with the repeater, and did not use it after the back was repaired.
We use all PCs in the studio, with a total of 3 desktop systems and a file server. Since the first failure over 2 years I have been shooting directly to a 32bit XP machine, without too many problems. About 2 months ago, all of a sudden the back would disconnect after taking the first shot, and would not reconnect again. I tried to tether it to the other systems (64bit) via Capture One, and kept getting error messages. So once again I sent it in for repair, and both the Firewire board and Ram board had to be replaced ($1250). A week after receiving the back from repair, it started doing the same exact thing. This time when I sent it for repair, I was told it's a PC issue, and since Leaf no longer supports the PC version of LC, I'm sh*t out of luck. At this point I got pretty fed up, and bought a MacBook pro, specifically for capturing the images and transferring them to the file server.
So far it has been running solid, with LC 11.3.6, on the Mac, but it's only been a week.
It is pretty sad however, that I have to spend $1,600+ just to be able to shoot tethered reliably. Not everyone in this business works on Macs!