I carry a LED Mini MagLite on my belt, and use it all day long. Recently I dropped one that landed hard and broke so had to replace it.
After a few weeks the new one started working intermittently. Progressively got worse and I replaced it too. Moved the fairly new batteries into the newest light and it was intermittent just like the one I was replacing.
Turns out it was something flaky with the batteries. New batteries and they both work great.