Lets just call it NFF, we can't spend forever testing it. After the spares pool is polluted with intermittent boxes, NFF rates rise rapidly. Operational Readiness suffers greatly. Avionics systems are replaced or upgraded. All new training and testing needed for these new systems.
New aircraft replaces old.
Engineers have tried every diagnostic ploy imaginable except one, Directly testing for the problem. Some don't believe that intermittencies are even a problem. They didn't do the math. In multi-level, ATE determines if the unit is good. Common sense tells us that if we have never properly tested the connectivity elements, then, over time, we may have created a niche for these latent failures to accumulate.