Got the same problem on my daughter's SD with a large convex mirror mounted on the original. I took it off and put little dabs of 5 minute epoxy on the ball, enough to cause more friction, not to glue the ball to the socket. Seems to have worked pretty well.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13