Using G5 DP 2.0 (1.5GB RAM) to capture DV footage to my local fileserver over gigabit to humangous raid5 storage (with blackmagic speed test I get about 60MB/s) the capture drops frames @ totally randoom points - max it captures is about 8min
Now I did the same thing from my mac mini over 100ether and had no dropouts for 12x60min so what the F**** gives
The server side is Opteron with 1GB mem 2.8TB raid5 intel pro NIC (dual port on PCI-X) inside the box blackmagic speed test gives about 160MB/s
If the NIC's are explicitly set to either 100 or 1000 or 1000/half and NOT Auto then it sounds like something else is going on in the background on the G5. Make sure all NICs are set to the same speed - don't use Auto. Check that you have no maintainance apps etc popping up.