I was service engineer for our company that supplied mini-computer-based control systems for large factories. We were on 24/7 call, as a failed control system costs thousands per hour in lost production. One night I received a call between 2 and 3:30 am for a couple of days in succession. Every night I would run the diagnostics for an hour or more - nothing - all within spec. Next night, the same thing, but suddenly the problem disappeared. Relief, but that nagging feeling remained. I checked the power feed, swopped the power supplies, CPU, - nothing worked. During shutdown periods I ran the diagnostics for hours on end - all OK.
A few weeks later the problem re-appeared. Any time after midnight I would receive a call…. Each computer had its own water cooled air conditioner. Then the penny dropped. When you get called you open the cabinet, switch off the air con and start testing… Aha, the unit heats up and starts behaving.
It was the start of winter, and during the night the water temp drops, the air con becomes more efficient, dropping the temp just below the failure point. I adjusted the air con up a degree or five till the next machine shutdown, and then dropped it minimum. Within an hour the co-processor bombed out. Problem solved and I could sleep again on my beat.