Many years ago I got called out to a mysterious issue that was bringing down a workshop's servers every Monday morning at about 08:30.
Systems were checked; cables replaced; even one of the power supplies changed, all to no effect. The environment was checked and it was established that nothing out of the ordinary was happening at that time.
Eventually it was decided that I had to go and spend a Monday morning on site to try and catch it red handed. For the first hour everything worked normally and then silence. After a few moments the servers came back to life and started rebooting.
It turned out that the workshop used a lot of air tools and had a huge compressor tucked away in a shed outside. Usually this cut in and out throughout the day with no difficulty to keep the air reservoirs topped up.
Over the weekend the ancient grease in the compressor's bearings essentially set, stalling its electric motor until it overcame this resistance, and stirred the grease back to a normal thickness - which it maintained until the next weekend.
During this period the supply voltage on that one, somewhat flaky, circuit dropped to about 30V, and needless to say, the servers were on the same electrical circuit.
My boss quoted for a complex solution involving UPS and the like, but the workshop simply oiled the compressor instead ...