login.def is present but empty. My thought process could be faulty, but the user logins that are sporadically failing have user ID's that match the ID's assigned to machine accounts - see excerpt from passwd file:
dlarson:x:593:711

eb Larson:/home/dlarson:/sbin/nologin
pbechik:x:594:710:Pam Bechik:/home/pbechik:/sbin/nologin
lwestberg:x:1205:710:Lara Westberg:/home/lwestberg:/sbin/nologin
atallon:x:1206:700:Angie Tallon:/home/atallon:/sbin/nologin
sjohnson:x:1207:703:Shelley Johnson:/home/sjohnson:/sbin/nologin
comsw05$:x:1207:300:workstation:/dev/null:/bin/false
Note how the UID's are 593, 594 and then jump to 1205, 1206, etc.? The machine account comsw05 also has uses 1207 and the user sjohnson/UID 1207 is the most recent user to encounter the failed login problem. I neglected to mention in my original post that the "permanent" resolution I've come up with is to add a machine account line similar to what you see for comsw05 with a UID that matches the actual user profile for that specific machine. For example, in the passwd file I've added the line:
is002$:x:1206:300:workstation:/dev/null:/bin/false
which ended the problem from occurring for user atallon/UID 1206.
Maybe I have more then one problem on my hands and can't see it because I am unfamiliar with the O/S.....