The problem with X-mas lights is not unusual, The effect is refered to as REIN or repetitive electrical impulse noise. Any thing electrical produces some radio noise/s. Its just a question what frequency, type of noise, & strenght.
If you suspect there is something interfering with your broadband , get a am/mw radio & tune it to 612Khz. if you hold the radio next to an lcd screen for your pc as an exsample you would hear a distinct noise. This should fade away if you move the radio a quater to half a meter away. Hold it by your modem/router & you'll hear the DSL signal.
If you get a distinct noise enveloping a larger area, then this may be picked up by your router & causing an snr problem (or even drop of sync), By using th radio you may be able to get an idea of where the noise is comming from. Switch the suspect appliance off & retest your DSL connection.
Be aware picking up a noise on the radio is not always aftecting your DSL connection, and you may still have REIN issue in the area which will not be picked up @ 612Khz, REIN is often notoriously difficult to pin-point.
X-mas lights are a classic cause, noisy elecrical appliance with a long length of wire (now an antennea). Some times you could cure it with a ferrite sleeve (that small cylindrical thing you normaly see along your monitor cable whichdosn't seem to do anything) You can get these from electical suppliers such as Maplin.
Last full blow REIN fault I worked on affected at least 3 customers within an 80m diameter, eventualy found it to be caused by a LCD monitor for a pc, this sent a strong REIN signal feed back along the mains cableing affecting the local area
Hope this helps..
