Windows 98 networking with Windows 2000 server

We have a setup of windows 2000 server and all client
machines are windows 98. 2000 server provides dns and
dhchp settings for client machine. The problem we are
having is that each time user boot up the machine and put
their login and password, sometimes it logs you in
straight but sometimes it gives message that password not
correct or no server available. After rebooting the
machine few times then everything seems to be working
fine.

We are just using TCP/IP on the server and all the client
machines. We don't know what could be the problem windows
98 machines not finding the server on boot up and why it
seems to find it after two or three reboots?
Any suggestions regarding this? any help would be very
much appreciate.
thanks
kashi