DURING A CROSS COUNTRY FLIGHT, THE OWNER/PILOT AND AN INSTRUCTOR PILOT WERE DESCENDING THROUGH A HOLE IN AN OVERCAST AT 135 KNOTS. THE V-TAIL DESIGNED AIRPLANE SUDDENLY EXPERIENCED 4 VIOLENT PITCH OSCILLATIONS. THE INSTRUCTOR PILOT, WHO WAS FLYING THE AIRPLANE, REDUCED AIRSPEED AND LEVELED THE WINGS. THIS ACTION STOPPED THE PITCH OSCILLATIONS. A POST FLIGHT INSPECTION REVEALED DAMAGE TO THE REAR FUSELAGE BULKHEAD, WRINKLING OF THE EMPENNAGE SKIN AND LOSS OF PAINT. THE AIRPLANE DID NOT HAVE ANY PLACARDED AIRSPEED RESTRICTIONS OR A VERIFICATION OF PROPER WEIGHT AND BALANCE CALCULATIONS. THE OWNER DID NOT PROVIDE ANY DOCUMENTATION OF COMPLIANCE WITH THE AD, OR OF PROPER BALANCING OF THE LEFT RUDDERVATOR THAT HAD BEEN PREVIOUSLY EXCHANGED AND INSTALLED. THE MANUFACTURER REPORTED THAT THE EMPENNAGE DAMAGE WAS TYPICAL OF RUDDERVATOR FLUTTER.