[Leica] is it possible for one M lens to misfocus?

[shino@ubspainewebber.com]

i have an m6 and the following lenses: (dates are design year, as per Puts)
  2.8/21 Elmarit-M 1980
  2.8/28 Elmarit-M (III) 1979
  1.4/35 Summilux-M, 1961
  2.0/90 Summicron-M (III), from 1980

i used to think the 28 was the sharpest of these lenses, but lately
i've been surprised to see images made on this lens have looked very
mushy and soft.  from examining some of these images, i suspect
that the 28 is focusing well ahead of the rangefinder.  my other
lenses on the same camera seem fine, even the 90.  if the m6's
rangefinder were off, i'd expect images on the 90 to be affected the most,
but my photos with the 90 are pretty well dead-on focus.

so, is it possible on an M that the focus on one lens is off?  i suppose
i could give the whole kit over to leitz, but realistically
that would mean being without the stuff all summer (shudder!).

any advice to a guy who may need to use an F3 all summer?

thanks,

- -rei