Blinking green LED twice a second

I have a PC plugged into one of the four Ethernet ports.  There is no internet connectivity on the PC.  The LED is not the normal flickering but a solid on / off about twice a second.  When I plug that cable into a switch ( that is in one of the other ports of the four), the PC connects normally.  When I take yet another cable from one of the four ports that is connected with a normal green flickering and plug it into the first problematic port, it started the on/off flashing.  Am I missing a logical troubleshooting step?  I have power cycled the modem a few times.  Anyone else think I have a dead port on my modem?

Thanks in advance.

gubbie and misha