Sporatic logins

I service a 28 school network.4 of the schools are suffering log-in problems. Nt4 servers and a mixture of 95 & 98 workstations.Running DHCP.Sometimes different people can’t log in. You might be denied and 10 minutes later able to log in.This includes my administrative account.Error says server not available to validate account or sometimes invalid password.These have been stable networks for 4 or 5 years.Exactly what protocol takes place between workstation and server at log in request? Alwaysable to ping successfully. Have internet access, but not able to log on to lan.