To know for sure we would need to see the topology of the connections between the host and computer being backed up.
But from your description, I assume the router in question is in the layer 2 path between the two ports involved in the communication. Check the data sheet for the router for the bandwidth limitations. You may be hitting a maximum for the device.
The other thing to check since the limit is at GE level is if the path from port to port passes GE only restriction some where. Or even a GE trunk port there this traffic plus the other traffic at the time is over capacity of the link as a result.
And as noted above look for error counters on all the interfaces in the path to confirm there are no physical media problems.