Recall reading a story by a network engineer some years ago - similar problem but at about 3 pm.
Days spent checking drivers in entire office network, all cables tested and so on, no joy.
Defeated team retire to conference room for a cuppa in the mid afternoon to discuss the problem; tea trolley comes through the door and the network crashes!
Turned out some lazy sod had laid the cable under the carpet outside the door instead of running it round the door frame. Cable rerouted and problem solved.
A long shot, but does anything physical happen around that time ('er indoors putting the cat out, etc)?