How do I find intermittent electrical faults I can't catch?

You can diagnose most faults now, but the intermittent ones beat you because they refuse to show up when you're looking. That is genuinely the hardest kind of problem in the trade. It is not a gap in your skill. It is the nature of a fault that hides. The thing that won't repeat on command has to be caught in the act. That means changing your approach from testing to monitoring. Instead of hunting for it while it's asleep, set things up to record what happens when it wakes: leave a meter logging, wiggle-test connections under load, look for what's common to the moments it fails. Heat, vibration, moisture, movement. The pattern is usually in the conditions, not the circuit. Next intermittent job, before you chase it, ask the customer exactly when it happens. Cold mornings, over bumps, when something else switches on. Those clues narrow where to watch. You catch these faults not by being cleverer in the moment, but by being patient enough to be there when they finally show themselves.