DEN83LA054

While en route, the plt encountered freezing rain and climbed to aprx 3500 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 aprx 1700 to 1900 ft MSL. As he descended thru 2500 ft, he placed 2 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 acft 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 alt air door parts were too badly damaged from impact to be operationally chkd.