On hard turns, my mirror always does that. And sometimes, even just under small bumps. Also, I find that on hotter days, the mirror "falls asleep" easier than it does on colder days. ??? Maybe there's a lubrication between that ball on the arm and the mirror casing that breaks down into a more liquid state because of the heat allowing it to move easier. Iono, that's my only logical explanation. One of these days, I'll try and find the fix for it.
iv personaly never had this problem but then again my car duznt have a system, and itz been in and outa auction yards, police impounds and what not for pretty much itz entire life. but i have a stock system and my outside mirrors shake with that whimpy bass i have :\