UPDATE: a little over a month and almost a thousand rounds later (different ammo), I disassembled and fully cleaned the gun, including the firing pin channel. I also replaced a number of springs, including the mainspring. No more problem. I did notice a difference in the trigger pull, especially DA. I have no means of quantifying though (what is needed to measure trigger pull anyway?). While there is no way to be 100% certain after so many variables changed, I'm guessing someone before me may have installed a lighter mainspring in an attempt to lighten the trigger pull. The spring itself was noticeably easier to compress by hand. Anyway, I didn't follow up and I wanted to thank rcmodel for the excellent advice!!!