Intermittent LAN

Installed ECS P4VXASD2 mobo with intel 2.0Ghz chip on WIN98 system. The LAN connection will intermittently fail after 20-30 min of use. All other PCs on the LAN are able to access the router and print server. The system will begin to operate after 30-50 minutes or after a reboot of the system and router. I believe it is a software issue but can find no reference on any of the sites. Any help would be grealy appreciated. John