I'm having similar issues with RHEL and CentOS as well.
In my logs, I'm seeing messages like this, over and over again, every 10 minutes. It makes me wonder if this is part of the problem:
[2008/10/15 20:26:44, 0] lib/util_sid.c:string_to_sid(242)
string_to_sid: Sid S-0-0 is not in a valid format.