Occasional log in issues, some machines, not others

Some of my users are getting the dreaded "Unable to log in as user "X" at this time. Logging into account failed because an error occured. The home folder is located on an AFP or SMB server. Please contact your System Admin" message, usually in the early morning, when most of them are trying to log in at around the same time. This only seems to happen on a handful of machines scattered around my building, and has been pretty much driving me crazy trying to diagnose the issue. The only helpful tips I've found in the Apple d-boards are "check your DNS" and that I should check my "network performance" -- generally, if the users wait a while, they are able to log in later. This makes me think there is something going on with my network, ie: network traffic spikes, since I believe a lot of users are trying to all log in around the same time. I heard from another sys-admin that he'd seen a fix for 10.3 that had you edit the LoginWindow StartupParameter.plist but unfortunately I can't find that file in 10.4as it isn't in the location the help specified (/System/Library/StartupItems/LoginWindow/StartupParameters.plist) - is this still used in Tiger? I'm using a XServe as OD Master (LDAP/authentication), DHCP and DNS, and have three XServes that house my user home directories - I have 600 users in my LDAP directory, and about 350 machines (mostly eMacs, some G5 and Intel iMacs) in use. The servers are running 10.4.10, as are the clients. Does anyone have any suggestions for where I should start besides RTFM? Any help is appreciated - thanks Blair