[fintstone]

I would bet it is the fixture or a connection in the light box. The only time I have seen otherwise is when a nail or screw has been driven through a wire. Sometimes that does not show up immediately. I had one house where they did this apparently when installing cable. The installed cable/outlet shielding was hot (at 110V). I found out when disconnecting the cable and was shocked. They managed to put a screw right through a live wire when they mounted the cable box outside.

I had another house where the previous owner put a large nail in the finished wall of the garage to hang something on. He drove it into a plastic drainpipe from a rarely used bathroom upstairs. The pipe was not under pressure and the nail largely filled the hole, so it only showed up now and then occasionally and looked like the roof leaked. Patch the roof and it seemed to go away...but always came back. Had to open the wall to find out the problem.