Q: Driver's side mirror moves straight up on its own

While merging in traffic on the interstate, I first noticed that the driver’s side mirror was pointing downward. While accelerating, I adjusted it. Once the mirror was adjusted properly I switched to the central neutral position using the mirror selector switch. Immediately after doing that, the mirror started moving on its own and pointed straight up. I barely missed a car in my blind spot (and there have been several more near misses, even though I'm aware of the problem). The dealership disconnected the battery and left it that way for a few minutes and then sent me away. It only fixed the problem for a few hours, but it always comes back. Now, “both” side mirrors have begun to do the same thing, but the driver’s side is the most problematic. What can I do to fix this permanently?