The flight instructor was demonstrating a power off stall during a training flight in a Cessna 172M. He observed the radio lights flicker and noticed that the wing flaps were "coming down more slowly," than normal. He "recycled" the No.1 radio, and while reaching for the master switch to confirm that it was on, the engine lost power and the propeller stopped turning. The pilot then lowered the nose of the airplane and executed the emergency checklist. He selected a field for landing and briefed the student and landed. Upon landing the nose landing gear collapsed, and the airplane nosed over, damaging the firewall and wings. A post accident examination of the airplane and engine by Federal Aviation Administration inspectors, revealed no preimpact malfunctions