I have a M686 that started doing the same thing. A couple of years ago I installed a Wolff ribbed mainspring. Gun never missed a beat until about a month ago. I was going to replace the spring but first tried the old primer with the guts removed trick. since then I've put around 800 to 900 rounds through it with no more issues. Not sure why it started with the light strikes all of a sudden but at least for now the primer trick seems to be working.