WITNESSES OBSERVED THE AIRCRAFT FOR APRX 8 MINUTES IN ERRATIC DIVING, CLIMBING, AND STEEP-TURNING MANEUVERS WHICH CULMINATED IN A 12 DEGREE DESCENT TO THE GROUND. THE SOUND OF THE ENG WAS HEARD THROUGHOUT THE SEQUENCE AND OTHER EVIDENCE INDICATED ENGINE POWER AT IMPACT. THE AIRCRAFT STRUCK TREES, A FARM BLDG, FARM EQUIPMENT, AND A VEHICLE, THEN BURST INTO FLAMES. THE WRECKAGE WAS STREWN ALONG A 320 FT PATH. NO EVIDENCE OF FLIGHT CONTROL MALFUNCTION WAS FOUND. THE PLT CLAIMED AN UPSET STOMACH BEFORE THE FLT. THE PILOT'S SPEECH WAS SLURRED IN RADIO TRANSMISSIONS. THE PLT DID NOT RESPOND TO THE LAST RADIO CALL FROM SIOUX FALLS TOWER. NO EMERGENCY OR DISTRESS CALLS WERE RECEIVED FROM THE PILOT. THE POSTMORTEM EXAM OF THE PLT DID NOT IDENTIFY SPECIFIC IMPAIRMENT BUT WAS LIMITED DUE TO THE SEVERITY OF THE IMPACT AND FIRE DAMAGE.