I thought I'd give an update on what the problem was. It's only been a year and a half! I did try replacing springs and the problem persisted. Through the process of swapping out individual parts from my good carbines it came down to the hammer. Different hammer, different (good) results. I bought a hammer at a gun show and it's been working fine ever since. Darn if I can tell a difference between the 'bad' hammer and the 'good' one. Sorry it took so long to update.