PILOT CALLED FSS ON WRONG FREQ. ANOTHER PILOT ANSWERED & PILOT REQ FSS FREQ. HE THEN CALLED FSS, RPTED ENG QUIT. WITNESS HEARD 'ENG STARTING' MOMENTARILY OVERHEAD, WATCHED ACFT PITCH OVER, DIVE TO GND & IMPACT WITH NO ATTEMPT TO FLARE FOR LNDING. PITCH TRIM WAS FULLY NOSE DOWN, THERE WAS EVIDENCE OF PROPELLER ROTATION UNDER POWER AT IMPACT, & NO REASON FOUND TO PREVENT ENG RUNNING. PIC BACKGROUND CHECK SHOWED TREATMENT FOR DEPRESSION, CONTINUOUS USE OF VALIUM TO SLEEP SINCE 1979, NERVOUS BREAKDOWN OF FATHER, FEAR OF NERVOUS BREAKDOWN HIMSELF, SEVERE SHORTAGES IN GOVT GRAIN IN HIS WAREHOUSE, & 1 MILLION DOLLAR INSURANCE TO WAREHOUSE. WHEN GOVT AUDITOR ARRIVED UNEXPECTEDLY, PIC LEFT WAREHOUSE, RENTED ACFT, TOOK OFF HURRIEDLY, & SUBSEQUENTLY CRASHED. WITNESSES REPORTED PILOT DID NOT PREFLT, DID NOT ACCOMPLISH A RUN-UP, AND TAXIIED AT HIGH SPEED.