BEFORE THE ACDNT, WITNESSES OBSERVED THE ACFT CIRCLING IN A VALLEY BENEATH A LOW CLOUD LAYER & BELOW THE SURROUNDING HILLTOPS. THEY ESTIMATED THE CEILING WAS 400' OBSCURED. TWO WITNESSES SAID THE ACFT ENTERED CLOUDS BEFORE IT CRASHED. A 3RD WITNESS, WHO WAS A PLT, SAID THE ACFT WAS AT A VERY LOW ALT WHEN IT APCHD RISING/MOUNTAINOUS TERRAIN; HE SAID THE ACFT THEN ENTERED A STEEP CLIMBING TURN & STALLED AFTER MAKING 2 COMPLETE TURNS. IMPACT OCCURRED IN A NOSE LOW, LEFT WING DOWN ATTITUDE. NO PREIMPACT MECHANICAL PROBLEM WAS FOUND. THE FAA HAD ISSUED THE PLT A SPECIAL 3RD CLASS MED CERT AFTER HE HAD QUINTUPLE HEART BYPASS SURGERY. HE WAS RPRTD TO HAVE GOTTEN 'QUITE ANGRY' BEFORE THE FLT, WHEN 2 PAXS WERE LATE. A PATHOLOGIST BELIEVED THE PLT'S DEATH MAY HAVE BEEN DUE TO ARTERIOSCLEROTIC HEART DISEASE; HOWEVER, THIS WAS NOT VERIFIED. THE PLT DID NOT HAVE AN INSTRUMENT RATING & NO RECORD OF A PREFLT WX BRIEFING WAS FOUND.