How do I solve a problem I can't reliably recreate?

You can track down most problems. The ones that break your spirit are the ones that will not show up when you are watching, that appear and vanish and leave no trail. Those cost you days and, worse, they make you doubt yourself. But understand: the difficulty is in the problem's nature, not in your ability. A thing you cannot reliably reproduce is hard for everyone, always. The trap with these is that you feel you should just solve it by thinking harder, and when you can't, you take it as proof you are slipping. You are not. Intermittent problems are not beaten by insight; they are beaten by patience and evidence. You cannot catch what you cannot see, so the work shifts from solving to watching, from being clever to setting traps and waiting. So stop trying to out-think it in one sitting. Instead, set up a way to capture more information the next time it happens, then let it happen and gather what it leaves behind. Turn the frustrating wait into deliberate evidence-gathering. You are not failing to solve it fast. You are doing the slow, patient work these particular problems actually require, and that is its own kind of skill.