>Then I'd look for something that happens sporadically, around the same time as this stuff happens.
>That could be anything from a staff member who comes in only periodically and uses an otherwise never used machine to
>... the floors in the department are heavy-cleaned around that time to
>... shipments arrive and the freight elevator is used around that time to
>... anything goes.
>I've heard of this kind of thing when something electrical is used randomly (periodically) and some hub or even just network cable is affected by current now running in a very (too) close line.

Yep, I've seen at one customer's site that their lights used to flicker when the phone rang! They had all kinds of wierd computer problems until they installed UPSs on all of the computers. They were only getting 103 volts out of the wall on good days.