I think it's mostly a symptom of him always being ready to run if need be. QBs that know they will never take off can use their legs differently than if they think there's a chance they'll take off and sprint. Obviously you're going to set your feet differently taking off on a sprint than throwing. I think Cam's "streakiness" with his feet is just him setting his feet right on plays he knows he's going to throw it with very little chance of taking off with the ball (e.g. quick slants or plays where he's going to throw it whether the target is technically open or not) and he mostly uses arm on plays where there's a greater chance of him having to take off with it (e.g. long developing plays where he's not sure someone's going to get open).
This is exactly what I am thinking. Remember at the beginning of the season when all the experts and fans raved about how Cam had finally taken the next step? Guess what, he couldn't physically run well. The second he was able to take off all of his bad mechanics returned and old(bad) Cam came back.