Inspiron 5100 laptop in full time ac power mode and  used virtually as desk top shuts down/powers off completely, on it's own.  The problem began ten days ago and continues one to three times a day.  Will power up immediately when prompted.  Length between episodes can be anywhere from 5 minutes to eight hours.....averages 3-4 hours.  Sometimes in middle of an application.....any application.  Other times when not in use at all for hours and on desktop with screen saver on.  Connections all seem good but the power source to the back of the unit doesn't lock like it should and can easily pull out.  The unit is a year old and not over used at all.  There appears to be no pattern to the problem at all.  Any clues or ideas would be appreciated.