I'm guessing it is something else, but you aren't using docker by any chance are you? Another API we use that is hosted using docker had containers keep falling over for no apparent reason. It turned out that it was because docker needed updating. It started happening early this year/very end last year too, so the timing fits which is why I thought worth suggesting, but doubt it will be the same thing.
Good luck on the fix, but please do keep us updated.