Loss of outgoing network connections.

I have a five computer network all running Windows 98SE. One machine is setup with two hard drives and the second drive is being mapped by all others as drive E. After two to three hours of being powered up, the machine does not show any other PC in Network Neighborhood and gives the message “unable to browse network”. The machine was recently formated and reloaded with Windows 98SE from Windows 95. While running Windows 95 this problem did not occure. All sleep modes reffering to the network card are disabled. I did cahnge out the Network card already with a known good card with no progress. Any ideas?