Domain field missing every Monday

Every Monday morning, 2 of my users boot their Win 98 computers and do not get the domain field listed on their login screen. After several reboots, the field appears and they are able to login to the network. They both get their IP address from aDHCP server and they are reserved just for them. This apparently begun happening when their office area was condensed down from 5 mini-hubs (one in each cube) to 1. Each computer is now connected to its own port in their own cubes. Anyone seen this before and have a solution?