At approximately 2115 mountain time, a Mooney M20C suffered a gear collapse while taxiing for take off at Boise Air Terminal/Gowan Field. The pilot and passenger were not injured. The pilot said he was taxiing for takeoff and approaching the end of the taxiway prior to turning onto the runway. He reduced power and the nose gear and left main gear collapsed without warning. The pilot also reported that the nose landing gear had collapsed after engine shutdown about two months earlier, but the reason for the nose gear collapse had not been determined.