Thread: The ever reliable Stirling Model 20.. why won't they work?

A question for Stirling enthusiasts/masochists & repairmen...

In my quest to find a Stirling m20 that fires more than half a magazine without stopping, I am now on my 4th model 20, and still no closer to the holy grail.

The problem with 'Number 4' is that it will fire the first round, and cycle the second round cleanly into the chamber without tripping/engaging the sear, so the firing pin is not locked back and thus will not fire. It has burst fired a couple of times because of this.
While cycling the bolt slowly by hand, it is possible to engage the sear and lock the firing pin back every time, and get the satisfying click of the trigger releasing the firing pin. But when pulling the bolt rearward and releasing it fast, the bolt skips past the sear and the firing pin stays forward.
(Rifles Number 1, 2 and 3, never did this, they always engaged the sear but always usually failed to feed properly, jamming live rounds or stovepiping empty cases.)
I compared the bottom of the firing pin bar, and there is barely any visible wear compared to others.
I cannot figure this out.
Anyone run across this problem in a Stirling before?

If I can't find a fix for this... anyone want to buy Stirling model 20 parts? Most of it works fine!