Motorola WR850G wireless router loses wired connection.

I have a strange problem with the Motorola WR850G wireless router. The
wireless part of it works fine. The problem is with the home pc that
is connected to it by the RJ45 jack it won't work at all, yet the
wireless is still working perfect. It's not the computer because my
d-link di-614+ works perfect. I have tried other network cards with
different chip sets. I have reset the ip on the wired computer. I have
changed RJ 45 cables. i have the updated the firmware.Tech support
tries everything they can think of and tell me to exchange for a
different one. This is the third router all with the same results.
Help!!!
Thanks,