Jim,
One thought is to check the trigger pull as SKB suggested but you may not need to change the sear angle, it could be that the Sear Spring (which is usually a Vee spring) may be weak allowing the sear to get bumped off with heavier loads. This would just necessitate replacement of the sear spring. Even a quality gun like the Simson (I have a couple of Simson Drillings) can have a bum spring from time to time.
Also, since you are in Minnesota, I have a buddy that has uses someone up around Superior who specializes in older SXS, Drillings and German Doubles. I will get you his contact info and forward it to you.
Don
