It sounds like the machine is falling out of ePO and re-adding itself. Whent that happens, depending on the policies you have configured, it couldlose its user assignments.
Search your audit log for one of the machine names in question and see if it was deleted and, if so, what deleted it. I suspect you will find a weekly scheduled machine cleanup stask that is the root cause of your issue.
Message was edited by: jim.roberts on 10/24/13 11:45:46 AM CDT