Desktop lost DNS settings

Hi,

I have a strange problem.

Some desktops, suddenly cannot reach a server.
DNS not working. I ping the server hostname, and there is no reply. if I ping the IP address I get replies. But I do can ping other computer using hostname.

Then if I go to the network settings and I just remove and add the DNS server IP address,(like a refresh) then I can ping the server hostname. What is causing this to happen? this happen in several desktops, but not on the same day. Like 4 of them work fine, but 3 other desktops have that problem. when I change the DNS settings, those desktops start working fine for the rest of the day or some days, until it happens again.

What is causing this to happen?