I had a similar case in the 70-ies:
Almost every day an IBM 360/65 was crashing at 7:30 +,- few minutes. Nothing in log, any other indication. My usual starting time was 08:00 but I decided to start earlier and to watch the machine. At 7:30 the cleaning lady moved with her trolley near the gate which contained the core memory of the mainframe and the system crashed.Next day I asked her to take another route and nothing happen. Most likely the reason was static electricity developed by the wheels of the trolley.