Is this profile shared by a user who may be disabling it by exceding the sign-on attempts?
We have some user that have macros set up to log them in and they forget to change them
when they are forced to change their password.
If this profile is not shared, you may consider changing the pasword to not expire and the number of signon attempts to something larger.
Is it possible there may be an unauthorized user trying to access your system?
If you are using the same profile to access multiple as/400′s it sounds like system value or setting may be slightly different for the profile.