I'm confused then, because your first post states, "Hammer started falling from the half-cock..." but then your post #5 states, "Holds at half-cock, will not hold at full-cock." To my mind, those are two separate issues.
Just for giggles anyway, try the experiment I describe in my last post. That will at the least rule out interaction between wood and sear, and will place the burden squarely on the lock itself.
If it is solely an issue with the lock, it's going to be very difficult to diagnose from afar. Firstly, I would disassemble and specifically look for wear at the nose of the sear, the nose of the fly and most particularly the full cock notch of the tumbler. Also ensure that the sear spring has adequate tension to hold the sear in the notches with the mainspring removed, then with the mainspring installed.