I have been having a similar issue recently (since January 17th) with my MacPro. Connections to specific IP/port combinations get dropped out of the blue while connections to other IP/Port combinations work fine. The IP/port combinations appear random and remain "broken" for periods of time (15 minutes to an hour) before fixing themselves. Resetting the network (unplugging/replugging) fixes the problem immediately. None of the other machines on my network have exhibited the same behavior.

I have changed ethernet ports, changed routers and am just about ready to reinstall Leopard. My setup had been working flawlessly before and the only change has been the update to quicktime 7.4/itunes 7.6/frontrow 2.1.2 on Jan 15th.

I sure hope it is not a hardware problem. Attempting to reproduce the problem for a technician is difficult. I wonder if the Apple Hardware Diagnostic software would find something.