Hello,
I've got a couple of servers in different locations and about 2 months ago, one of them was unreachable via the internet - thought it was an ISP issue. Once on site, I thought it was a loopback issue so I unplugged network cables until I found the one going to the file server brought down their whole network. There is no IP conflict with anything on the network, nor is there a mac conflict.
I tried changing the IP and that didn't help. I tried plugging in to another network port - no luck. I even had dell change the motherboard and it didn't help. The only solution which is not a solution is to remove the default gateway on the nic.