A CESSNA 152 NOSED OVER IN A ROUGH PLOWED FIELD AFTER A PARTIAL LOSS OF POWER WHILE ON A NIGHT VFR PLEASURE FLIGHT. THE PILOT INDICATED THAT HE WAS ON A DATE WITH HIS GIRL FRIEND AND OFFERED TO DEMONSTRATE NEGATIVE GRAVITY TO HER. HE SLOWED THE AIRCRAFT AND CLIMBED TO APPROXIMATELY 2,600 FEET MSL. DURING THE RECOVERY FROM THE RESULTING DIVE, THE AIRCRAFT ENGINE BEGAN TO RUN ROUGH AND WOULD PRODUCE ONLY PARTIAL POWER. THE PILOT CHECKED THE MIXTURE CONTROL, THE MAGNETOS, FUEL QUANITY, FUEL VALVE, AND CARBURETOR HEAT. THE ENGINE WOULD ONLY PRODUCE 1,400 RPM AND THE PILOT WAS UNABLE TO MAINTAIN ALTITUDE. THE PILOT EXECUTED A FORCED LANDING TO A ROUGH PLOWED FIELD. THE AIRCRAFT NOSED OVER DURING THE LANDING ROLL. EXAMINATION OF THE AIRCRAFT ENGINE BY AN AIRFRAME AND POWERPLANT REVEALED NO DISCREPANCIES IN THE ENGINE CONTROLS OR AIRCRAFT FUEL SYSTEM.