I have the same problem with the mirrors I bought from LMC. What I ended up doing was tapping a hole through the threaded part in the bent arm and putting a bolt through it to keep it from rotating at higher speeds. It's ghetto as hell, but it works... Ok, where is it... THERE!
They still shake and shimmy till hell won't have it... I'm also looking for a good solution. I'll watch the thread for other answers...