As the airplane was deviating around low intensity weather returns, and passing through 20,500 ft during climb out, it departed controlled flight and entered a right spiral. Descent rates exceeded 16,600 fpm during the descent from 18,300 ft to 3,900 ft and then slowed. Witnesses stated they saw the airplane descend from the clouds in a right flap spin. The empennage had separated from the airplane in pieces. Pieces of the wreckage were found up to 1.5 mi from the primary impact point. Both wings remained attached. The right engine was flamed out, and the prop was feathered at impact. No mechanical reason for the flameout could be determined. At no time did the pilot indicate he was having difficulties. The NTSB weather study indicated that moderate turbulence was present in the area. The pilot's toxicology tests found 0.151 ug/ml of chlorpheniramine in the blood; normal therapeutic concentration is 0.01 to 0.04. Effects of overdosage include sedation, diminished mental alertness, and cardiovascular collapse to stimulation.