>>After a long time, they figured out it was when a vacuum cleaner was being run or something like that.

True story: a customer complained that an app used for appointments would repeatedly crash with patients on the phone trying to make appointments. In the end I stood and watched the PC in use with one of the customer's managers, hoping that a fresh set of eyes might spot something. As we watched, a phone rang on an adjacent desk; as the telephonist leaned across to take the handset, her foot nudged a protruding overloaded power socket bulging with multiplugs, restarting her machine.