Just to eliminate a possibility based on the vintage of your bike.
If you have stainless steel spokes (identifiable by a shiny slightly yellowish appearance, and low magnetic attraction) check for small black inclusions (dots, streaks or veins) on the surface. If you see any grab that spoke and a neighbor and squeeze hard to flex the spokes. If it snaps, my theory is confirmed --- your spokes have cancer, or more specifically chloride corrosion. There is no cure for this, and it's terminal, the inclusions are incipient fracture points rendering the steel there about as strong and brittle than spaghetti in the box.
The ONLY option is to replace all the spokes, or buy/build new wheels.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.