Nic quits responding

I have several web site running on a single computer under iis. Over a period of time usually les thatn a day one of the NIC card to a site will stop responding, yet if you check the server it shows as being active. If i change the media type from auto mode to 10 MBS half or anything other that what it currently is, then it will start responding to web and ping traffic again. This problem is appearing on all of my servers at some point or another. Any idea what would be causing this to occur.