I finally found the problem (mentioned here) that was causing the left magneto on my Lycoming O-320 to be intermittently hot (active).
I had searched and searched for the occasional problem. Traced wires, tested switches, and so forth.
Finally, this past weekend, I was REALLY studying the P-Lead and magneto and the problem stuck me out of nowhere. When the P-Lead was firmly bolted in place, there was a bit of play that allowed the P-Lead wire into the magneto to move fore and aft about an eighth of an inch. This was causing the connection to be broken. It is this connection (of the P-Lead) that “grounds out” or disables the magneto. Without the connection, the magneto is hot or active all the time. This is very, very dangerous as someone could turn the prop and causes the engine to fire which can really do some bodily harm. This is serious stuff and definately not something to mess with.
I fixed it by adding a very short length of phenolic “tube” to the already existing piece. This phenolic is basically a spacer that pushes the wire onto the contact point inside the magneto.