Reminds me of when I did DSL customer service. Once every few months we'd get a customer (never the same one twice) calling up saying that their connection was terrible, and when we checked the logs, their connection would dropping at incredibly regular intervals aligned well with human units of time. On the hour (or a few minutes past) every hour, for example, or every 10 minutes.
We figured, and told them, that it had to be something on a timer causing an EMP or other interference at their end because there ought to be nothing outside their premises acting with such specific regularity.
The caller usually went away and we'd see the connection stabilise not long after, leaving us none the wiser, but hey, fixed!