Long start PC to Windows 2000 / urgent

The network MS Windows 2003 is after migration users a
computers account fomr NT4 domain. In W2k3 domain are 3DC
and about 700users with W9x, WNT, W2k PC. I've following
problem. Very small random number of people start their
PC about 15 minutes. In security log of users appear
records with the pause between Event ID 515 a 643 , eg.
during autentification of computer.Every authentification
logon of computer is success.
After that is implemented scecli, eg. security policy
and logon process of user.
Thank you for any idea.

Jan