Ping Problem

I have a Webserver that has 2 NIC. One address is internal 10.10.10… and the other is external 66.134…. When pinging from the server by name on the actual server, it sometimes pings the external and sometimes the internal. If it pings the external, the ping command will only work if I go to the system32 directory through DOS. If it pings the internal, I can ping from the C prompt without going to the system32 directory. This also effects clients from accessing our internet site internally. HELP!!!