Amazing, and exact version of this happened to me in the Israeli Army in the 90's.
Every day at around 13:30 (give and take a few minutes) the network for an entire building went offline, only to come back on line a few minutes later.
... In our version of this 'bug' - one of the NCOs unplugged the electricity to that "big box with lots of blinky LEDs" in order to plugin an electric pot, to prepare his post-meal Turkish coffee, then plug it back in once water boiled...
Many resources were "thrown" at the problem, until, weeks in, someone had the splendid idea of quietly sitting around the network-cabinet around 13:30 and see what's going on.
Every day at around 13:30 (give and take a few minutes) the network for an entire building went offline, only to come back on line a few minutes later.
... In our version of this 'bug' - one of the NCOs unplugged the electricity to that "big box with lots of blinky LEDs" in order to plugin an electric pot, to prepare his post-meal Turkish coffee, then plug it back in once water boiled...
Many resources were "thrown" at the problem, until, weeks in, someone had the splendid idea of quietly sitting around the network-cabinet around 13:30 and see what's going on.