spyder,
It's a long shot, but is there a home office in the residence? I've seen instances where a laser printer firing up caused voltage drop outs at about a 2 Hz frequency.
A laser printer draws a large amount of power about every two or three minutes when on standby to keep the eletrostatic charge up on the copy drum. For a good sized printer or copier, the load may be 30-50 amps at 240V for a few cycles. When printing, large heater coils come on to fuse the toner to the paper.
If there are two or more printers or copiers on the same branch circuit or on circuits fed from the same subpanel, you can see really complex patterns of voltage drop outs. I used a Fluke 189 to record branch circuit voltage and the Flukeview software on a laptop to analyze it. I'll try to post a couple of files in the next day or so.
Best of luck--
Cliff
