AD/DFS Replication between 03/08 boxes would fail approximately 15 minutes after the servers were done booting. Reboot and they would be fine for another 15 minutes.
Mine was physical problem... The network died. Nobody could connect to everything, although the switches were powered up, the servers and clients were all peachy...
Turned out a BSOD'd workstation brought the whole network down by generating a bunch of garbage packets. All the guy did was just start unplugging cables until we found it, then I went to that jack and found the BSOD. I felt like a moron.