I have the same problem -- it was happening over a year ago and now it has started again. It seems to be isolated to the events that are on the exchange server based calendar (although that may be just because that is the only one that has many invitations). It is not isolated to recurring meetings but is happening for many old meetings.
There used to be a setting for how long calendar entries were stored and seemd to happen when that date was reached, but that setting is no longer in preferences. (Version 6.0 (1645))
Any ideas? This is incredibly annoying to my clients & colleagues.