The helicopter was on a test flight to track and balance the main rotor system and to complete a 100-hour inspection. Prior to the flight, an occasional flicker of the fuel filter light was noted, but it was thought to be a wiring problem. After taking off, the fuel filter light flickered on a few more times, but never illuminated steadily. After less than a minute of flying time, the pilot felt two quick 'jerks' (yaws). He started a 180-degree turn to land. After 90 degrees of turn, the N2 decreased to 95%, then surged to 105%, then the engine quit running. The pilot initiated an autorotation, but during landing on soft, wet terrain, the helicopter rocked forward and the main rotor struck the tail boom. An exam of the helicopter revealed a 'B' nut on one of two differential pressure lines (connected to the fuel filter differential pressure switch) was loose. During the exam, the 'B' nut separated from its attach point by a half turn using finger pressure.