The sear lever.

Either it's very well fitted (and doesn't get stuck when it actually holds the lever) or it doesn't actually engage. I would be keen to remove it and see if it actually does what it is supposed to, but the risk of "stuffing up" this fine balance of operation (if it is working as it is supposed to) when I re-assembled it precludes me from doing this. The old aint-broke-don't-fix principal applies.