A marginal cable, especially a long one, will sometimes work at a lower frequency and not work at 100 MHz. The DI-704 Home DSL/Cable Gateway is a combination router and 4-port Ethernet switch (http://duxcw.com/faq/network/hubsw.htm). It should operate full duplex with a network adapter capable of full duplex operation. Assuming there is no flaw in the auto-negotiation process or the ports, half duplex operation could indicate a marginal cable. During the negotiation between the network adapter and the DI-704, a condition where packet collisions can occur is detected. This condition is most likely derived from corrupted packets or excessive noise (http://duxcw.com/digest/Howto/network/cable/cable4.htm) on the line. The result is that when transmitting the receiver at the port where the transmitter is located is set to listen for packet collisions instead receiving data simultaneously from the transmitter at the other end of the line.
I would not be satisfied with this condition because of the increased probability that there will be network problems (drop-outs, etc.) and data corruption, and the performance certainly will be marginal. The correct course of action is to verify that the cable is made from CAT 5 or 5e cable and plugs, that the cable has solid core wire (for cables longer than 3 Meters or about 10 feet; I have seen longer cables with stranded wireó25 feet--work OK), the plugs are designed for solid core wire, the cable is run correctly (http://duxcw.com/digest/Howto/network/cable/cable9.htm), and the plugs are wired and crimped properly (http://duxcw.com/digest/Howto/network/cable/cable7.htm and http://duxcw.com/digest/Howto/network/cable/cable8.htm). If all of these conditions are meant and the cable and plugs have been carefully inspected, flip a coin, cut the plug off one end of the cable, replace it, inspect it, and test the cable again. RJ-45 plugs are cheap; an unreliable network can be very expensive. Larry