DURING THE INITIAL CLIMB AFTER TAKEOFF, THE RIGHT ENGINE EXPERIENCED AN INTERMITTENT POWER LOSS. THE CFI TOOK CONTROL OF THE AIRCRAFT, ESTABLISHED A DESCENT TO MAINTAIN AIRSPEED, AND IDENTIFIED THE PROBLEM ENGINE. AFTER THE ENGINE WAS PULLED TO IDLE, THE PILOT REALIZED THAT HE WOULD HAVE TO ATTEMPT A FORCED LANDING IN A FIELD OFF THE END OF THE RUNWAY. THE TOUCHDOWN WAS SUCCESSFUL, BUT DURING THE LANDING ROLL, THE AIRPLANE STRUCK A COW, A FENCE, AND THEN A DITCH. THE POST-CRASH INVESTIGATION AND ENGINE INSPECTION WERE UNABLE TO DETERMINE A REASON FOR THE POWER SURGE/LOSS.