Its the neighbour in the flat above! I've had all manner of problems with line transients, one summer when I had the computer desk by the open windows I heard a loud bang from the flat above - at the same instant everything on the PC screen scrolled up off the top and froze, apparently the nutter upstairs had ripped the lead out of something, twisted the ends together and plugged it in! At the time I was servicing PC monitors & PSU boxes, so any scrap units were plundered for mains surge varistors. Now practically every socket in the flat has one mounted on the terminals inside the outlet box. Before this it was common for monitors on soak test to fail with S/C rectifiers - this also no longer happens.