SBS 2003 Connectivity issue

We run a Dell server with SBS 2003. 5 machines are hooked to the server. The server acts as our web & mail server. It connects to the internet without fail. The 5 user machines are having intermittent connectivity problems getting on the internet. The intranet seems to work fine. Pinging from one of the 5 machines will produce an error (cannot find host) about one in 5 tries, but it is inconsistent. No configuration changes were made on the system before this occurred.