Sometimes after fireing pressure can raise, the brass stretching to fill the chamber is a sort of "cushion" for the pressure, where as once it is stretched to fill the chamber there is no such cushion. But this usually comes with other pressure signs....
I have seen it happen specifically in belted factory remington rifles, the problem was a rough chamber, the solution was some jb bore bright on a snug fitting patch on the end of a dowel in a drill gun to polish it some, just the chamber. The brass had scratches on it though, going lengthwise with the brass. You don't want to go too much if you polish, as I have heard polishing a chamber too smooth can cause its own set of problems, the case can't adhere to the walls of the chamber during the firing process, and instead applies more pressure to the bolt face, showing early case head pressure signs, and in some cases can lead to bolt lug set back.
I know you have, but measure the fired cases (specifically the stuck ones) to ensure that they are round and not oval or show some other chamber deformity. You said the chamber was cut way too large, there is no limit to what else may be messed up with it.....
Just a few of my ideas where I would start.
