97 Mirror Problem

Within a 4 week period, I have had two mirror assemblies break on my 97
coupe. The problem seems to be with the spring detent that allows the
mirror assembly to pivot if struck. First, on the drivers side, and 4
weeks later, the passenger side. In both occasions, there was no apparent
cause or trauma that might have caused the problem. The car is kept in a
garage, and nothing has struck or hit either of the mirror assemblies, they
just suddenly were twisting freely. Has anyone else had a similar
occurrence or have any ideas as to what may be causing my problems?