Does it stop when you unplug or turn off the router? I'm thinking it could be something to do with interference coming from either the router or its associated cabling (which may or may not be shielded properly)

Also, the Quadra 650 actually only supports 10Mbps ethernet with its onboard ethernet connection - only way to get 10/100 is via a very rare 10/100 NuBus ethernet card. Thankfully however, unlike some third party ethernet cards (such as those put out by Asante), the Quadra's ethernet port has no trouble connecting directly with an autonegotiating network device and running at 10Mbps, which is a Very Good Thing™.