ADSL is a fickle thing, and unfortunately as you have found that it is a device in your home that is causing the issue any ISP will (if pushed) get an engineer out to look at it, but it will be a BT engineer, and they will charge you (BT will charge your ISP and they in turn will charge you) upwards of £80 just to tell you what you know..... The BT engineer will diagnose the issue, but as it is a problem with your equipment they will not fix it for you...just tell you what you already know.....
Drop-outs due to electrical interference is not that uncommon.....
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Having read the thread I posted, it looks like the likely culprit could be the power supply built into the monitor....and it appears that the best way to test is with an AM radio.....