I had the same thing happen with my 375jdj and I asked these guys what they thought and I got the same recommendations on the headspace and the hammer spring, I bought a headspace gauge and extra power hammer spring from bellm and after installing the spring and adjusting my headspace about .004 I havn't had a single misfire.

I won't be wronged. I won't be insulted. I won't be laid a-hand on. I don't do these things to other people, and I require the same from them. John Wayne-The Shootist