UK ISP PlusNet said:

"When putting up the tree this year, don’t forget that some Christmas tree lights (especially some flashing ones) can cause intermittent connection faults. So, if you think you’ve got a broadband fault please remember to turn your lights off for a while and see if that cures the problem before calling.

Some tree lights generate RF interference which can affect broadband ADSL. It’s best to keep your Christmas tree lights and wiring well away from your telephone cables.

Some people have suggested that one way of detecting interference is to tune an analogue radio to Medium Wave 612 through to 693, which should buzz when you wave the radio near problematical tree lights. We’ve not tried this though and you may have more than one source (RF band) of interference."