Mark, not familiar with Merkels or boxlock guns but I still think it is a sear problem. I agree to what Rainey states, disconnect the safety as I think the recoil has something to do with it. Reading up on a few sites the only thing I saw on this model was "doubling. I would contact Merkel and asked them.
Why it only happens after 12-15 shots is the $100 question.


David