As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE AIRCRAFT WAS ON A TRAINING FLIGHT IN A HOLDING PATTERN. THE ENGINE LOST ABOUT 400 RPM'S FOR ABOUT THREE SECONDS. THE INSTRUCTOR TOOK OVER CONTROL AND APPLIED POWER. IT WAS THEN THAT THE POWER RETURNED. SUBMITTER STATED THERE HAVE BEEN NUMEROUS UNEXPLAINED POWER LOSSES IN THIS FLEET OF 53 172R MODEL AIRCRAFT.