authenticating w98 in Mixed Mode

I upgraded my NT PDC to win2000 DC. Setup DNS and running in Mixed Mode with 3 NT BDCs. The network has most 98 machines and about 100 w2k machines. Everything running fine. Just found out one day when NT BDC#1 was down, no w98 machines can authenticate and logon to network. But w2k machines had no problem. I had to reboot the NT BDC #1, then w98 machines can logon again. Why didn’t NT BDC #2 and #3 authenticate when #1 was down? Why didn’t the w2k DC authenticate the w98 machines ? What shouldI look for and correct ? Thanks for any help.