I fixed mine a couple years ago by taking them apart and adding a simple rubber washer from a garden hose into the ball and socket joint.  This worked for about 2 years unil this spring when the mirrors started flopping again.

  Upon disassembly I discovered the root of the problem.  The 10mm nut holding the mirrors together attaches to a hex shaped stud that is press fit into the main mirror body.  Over the years, the press fit had loosened and repeated tightening of the 10mm nut had started to pull it out of the mirror.  I cleaned up everything, applied some red Locktite to the stud, let it dry overnight and re-assembled the mirrors.  They are now more stable than they have ever been.  Over a month's time they have not moved a bit.

  Clint