1. I had the same problem with my 390RB. I had loosened the hex bolt at the top of each mirror arm with a ratchet wrench to readjust the mirror shell angle, since I had run out of adjustment with the electric motor on the console switch. I didn't get the hex bolt tight enough after I completed the adjustment, because I was afraid I wouldn't be able to loosen the cone mechanism on a subsequent readjustment, and at 60 mph the mirror moved out of position.
2. The key to getting the mirror to unstick from its present position after loosening the hex bolt on top is to just grasp the mirror shell by the sides at the bottom and try to move it until it unsticks.
3. Then tighten it a little more the next time. There is an internal cone mechanism that can be overtightened with the hexhead bolt, so don't go too far.
4. If it still moves out of position at speed, repeat 2 and 3, above. Try not to overtighten, per 1 and 3, above.