a cracked/fractured #5 cylinder and subsequent failure/jamming of the #5 exhaust valve, which resulted in a partial loss of engine power; and failure of the pilot to maintain adequate airspeed, which resulted in a stall and collision with the terrain. Factors related to the accident were: the excessive gross weight of the airplane, the pilot's lack of familiarity with the airplane, and pilot impairment from use of an over-the-counter medication with sedative side effects.