Is it actually saying that it's disconnected, or is it going too quickly for you to read it? I'm not sure if the disconnect message is the only one that can appear.
Sounds like either the port or the cable or the remote port may be bad. Even though you're able to ping, the connection may actually be failing. If it fails fast enough, the link might be maintained because your system won't lose its IP quickly, and the switch would still have the MAC address in its ARP table (if it's used on a hub then it doesn't matter beyond electrical signal, MAC isn't even an issue).
Really no way to test it besides replacing cable, changing port on the switch/hub, or replacing the NIC. You could try uninstalling and reinstalling first, since that'd be the simplest thing.