Hank, I had a problem like that a while back. It was caused by a burr on my extractor, keeping the extractor from opening to grasp the case rim about 1 in 5 shots. I would get a misfire, and a very light pin mark since the case rim was not seated into the bolt face.
Do this next time it misfires: carefully pull the slide rod back, and see if the case follows the bolt. If not this is your problem. I fixed mine by very carefully stoneing by hand with a fine stone to remove the burr, being careful not to change the extractor angles. Never had another misfire.