IIS ignoring connections

I've got a Windows 2003 server (sp1) running IIS with all current updates
installed. Periodically, but not consistantly, it starts ignoring all
incoming requests. I have placed network sniffers on both sides of the
router serving this machine and have verified that the ethernet packets are
getting through to the machine. If I pull the cable from the port or reset
the router, then IIS starts responding to traffic again. The machine is an
older Intel quad Xeon server, the ethernet port being used is built into the
motherboard (PnP recognizes it as an Intel 82559). Does anyone know of an
issue with IIS 6.0 or with the drivers for this Intel port, that could cause
this symptom?