Does your gearbox sound like it's racking the piston all the way back before releasing it? A stripped / broken piston tooth usually sounds more obvious (depending on your experience with various gearboxes).
Make sure you're not having a double or mutli-feed. Remove, clean, and re-seat your bucking to try to fix it. Some times the bucking (made of rubber) can contract and expand while it's just sitting in storage year after year, through summer then winter; and the normal upkeep of cleaning and re-seating it doesn't take place.
