My guess would be outside interference of some sort. Possibly from some device that sends weekly monitoring updates via radio signal or even a weekly process using some machine that emits a strong radio signal. Some years ago, we used to get a 10-15 minute TV signal interruption at exactly midnight that we finally traced to a nearby river level monitoring station sending daily reports to a microwave tower. Our TV antenna was almost perfectly aligned in the narrow signal path. In our case, relocating the TV antenna pretty much eliminated the problem, but I suspect your interference could be from a much broader signal. Is there any sort of manufacturing or processing facility nearby?