I think you're already on your way to a solution... I'll add another possibility, just to confuse you :P
I used to have the same problem when my boots were a tad too big. I was cranking down the bindings as tight as I could and it was a lot more work, as I was trying to keep my feet from moving around in the boots, as well as dealing with heel lift.
Come to think of it, I still tighten the bindings just as much, but with proper fitting boots, it feels fine.