With RSL running, Right-Click on the graph area and select "Plot Upstream Noise"
If the noise is just a "pulse" then you may/maynot find it on MW.
Try to borrow another modem and trying it on just the incoming pair with nothing else.
There is something drastically wrong, somewhere and you need to prove where it lies - in your house, in the street or at the exchange.
One thought - when it next drops - immediately (Within a second or two) try to call your neighbours phone number and see if it rings/engaged.
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