This situation reminds me of a gremlin we chased on a B-52 years ago. One of the systems my shop maintained was the autopilot. Three electric motors in the system moved the manual controls, which were simply cables and bellcranks attached to the pilot's column and rudder pedals. One plane had a chronic stiffness in the controls which only happened at altitude. Back on the ground, or at low level, the problem went away. After weeks of swapping parts, one of the maintenence guys I supervised found a piece of wet rag wound up in an aileron cable sheave. At high altitude and below-zero temperatures, it would freeze and restrict the control surface movement. By the time we got the (repeated) work order complaint, it had thawed out, and the problem disappeared.
Jerry
