While en route, the pilot encountered freezing rain and climbed to approximately 3,500 ft to be on top of all weather. When he throttled back, the engine began surging and would not produce enough power for level flight. The pilot knew that the ground level in that area was approximately 1,700 to 1,900 ft MSL. As he descended through 2,500 ft, he placed two mailbags on his lap. He dimmed the cockpit lights, turned on the taxi lights and windshield alcohol, and began looking for ground references. However, he did not see the ground until the time of impact. The pilot believed that the automatic feature of the alternate air door had malfunctioned or failed. He reported that he and another pilot had previously had a similar problem when the same kind of weather was encountered. Reportedly, at one time, the aircraft had been equipped with a manual control to operate the door, but this feature had been removed. During a functional check, the engine operated normally. The alternate air door parts were too badly damaged from impact to be operationally checked.