it sounds like the trip point was right on the edge of being to close to the bolt closed position. the extra heat expanded things just atiny bit and it is now past that point. take a little bit of material off the top and back of the trip wedge on the top of the trigger to make the trip point a tiny but further forward. use the video at the bottom of the first post in the sales thread for modifying for non stock hammers.
its either that or some dirt, or a worn sear of bolt lip.
I can't see how the spring or gas pressure could cause it but I could be wrong.