You shouldn't be having any problems connecting W98 clients to a W2K server. I?d look for network problems. Do you connect the clients to other servers and they do not lose connection? If so, then the problem definitely lies with the server connection or network interface.
Maybe you have a problem with the hub or switch port for the server. Or you might have a problem with the NIC driver at the server. Do you have any network-related Event Log entries?
If none of this seems to explain the problem, write back with a description of your configuration and an example scenario when a client loses connection.