Were the Avos in particular and other biblical protagonists (e.g. the Imahos) regular people? That is to say, do we generally ascribe deep holy explanations for their behaviors or just say they erred? Looking for views of the G'onim and Rishonim in particular and to a lesser degree the views of Chazal.
The most famous case of a Rishon pointing out the human flaw of a patriarch is the Ramban's discussion of Avraham's descent to Egypt in Lech Licha 12:10. Rabbi Hirsch goes to great lengths to expound on that opinion.
The Medrash Rabba in the beginning of parshas Shemos has a list of our heroes who ruined their children by showing them too much love. Avraham Yitzchok and David made the list. This may or may not make your cut considering the Maharatz Chayos' assertion that the early Rishonim did not have it. I'm pretty sure Rabbi Hirsch mentions this medrash but I don't see it right now.
R. Avraham Ben HaRambam cites the verse "do not touch my anointed and do not harm my prophets" (Psalms 105:15) in the context of negative assessments of members of these categories. This would certainly include the Avos, (the prophecy of the Imahos on the other hand is not explicit in the Torah, so this "prohibition" isn't as clear.)