I would expect Stafford to have a much higher QB rating vs below .500 teams. The fact that he doesn't is an excellent example of how he is an inconsistent QB.
As a disclaimer, I like Stafford as our QB because of those flashes of greatness. But I would like him to be more consistently great. Will he have stretches of games where he isn't great, of course. Every QB does. The elite QBs do not have long stretches. Matt needs to make that next step in his career to elite. So far he is only good to very good, not elite.
Good to very good should read good to very good sometimes. Or have you forgotten 2012, bad. 2013 and 2014 average. First half of 2015, not so good. And the close of this year.