PCs dropping off NT network -HELP!

I have a Windows 95 and 98 computer as part of an NT4 network running DHCP and WINS. The problem I am having is that after an undetermined amount of time both computers “fall” off the network or seem to lose their ability to browse any other computers (Netbios names). I can still ping other computers. I have tried putting static DHCP information in, not having any apps open, logging in different users, different wall jacks, ect. I do not get stumped often, but I am now.