Well, Liane, that's the first time in the current shambles (I think) that anyone's mentioned keyword density, and it may be as plausible as any other theory.
The problem seems to me that everyone has equally compelling suggestions about what to do to put matters right - which indicates that there are many factors in play and that what would put one site right will have no impact on another.
Nonetheless, your comments about keyword density did strike some kind of chord. Has anyone else got proof that this is a factor of importance?
Meanwhile I'll do my own test and report back later.....