I have a linux box running RedHat 6.0 which is an e-mail server for 60 Win95 PCs using Netscape Communicator as a e-mail client. The linux box has its own gateway with a dedicated ISDN line and entries for our ISP's (ISP "A") DNS. All of the client's server settings in Netscape contain the IP address of the linux box so that the name doesn't have to be resolved. The clients who have internet access have a different gateway from a different ISP(ISP "B") with DNS settings for that ISP.
Most of the time the system works great, very fast and reliable. Sometimes, however, the linux box gets very slow both sending and retrieving mail, making telnet connections etc. When this happens, if I check the active connections with netstat there are multiple udp connections to the ISP's DNS (ISP "A"). After a while (sometimes hours) the connections will close and things go back to normal.
Can anyone suggest a reason for this and a possible fix. Any help would be appreciated.