I took delivery of a Dell XPS 8930 two days ago and have exactly the same problem. At random times the powerplan is changed from "Dell" to "power saver". The Powersaver plan puts my PC to sleep which is totally unacceptable. There are multiple error entries in the windows system log when this happens (Event ID 2000, task ID "Neversleep Agent Service", which mention something about SetPasswordRequiredValue, none of which I understand.
This would seem to be a Dell XPS problem. It has never been reported for any other brand, and the problem seems to have started recently with Dell.