How close is the fan to your cable modem? We had a problem with a monitor in work, its display would shake like crazy when it was a hot day. It took us ages to work out what was wrong with it.
We replaced the monitor - no joy
We replaced the base unit - no joy.
Found out it was a fan on the desk of the person in front of them. We couldnt see the fan because of the partition in the way.
Try moving the fan away from your cable modem and try it again, see if that works.
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