If you are finding unproffessionalism only in her eating and sitting habits, tell her directly. Tell her in future too, you point at her in case such things comes under observation
At times, it so happens, that just becoz that person is clumsy, we may feel that the person is not even good in work. Say for example, if she had good habits , would you have still commented on her performance,
try thinking on that line and rate her performance, if her performance is really bad, then find out why.
If she is lacking focus, try giving her one work at one time, tell her to just concentrate on that work and slowly she will dwell in to these habits and see if she improves. In case she does not, then one more option, you can think of putting her in other department ( ofcourse wherever she fits) .
I agree, it is difficult to predict someones performance, what i suggestm whenever you finalise some one, tell them to come two weeks training, based on their successful completion only they can be recruited. The canditiate can atleast take two weeks leave from their current company and will also not loose job.