I'd suggest posting your line stats here for everyone to take a look at. Preferably you should post some stats from daytime usage when everything is normal then stats from nighttime when you're having your problems.
Do you have any appliances that turn on at night that might be causing noise on the line?
I called AT&T during the day today (few hours ago) and currently the speed is... acceptable and they didn't see anything wrong from their end. They told me to call while the service is acting up again, so chances are I'll call them again in a few hours.
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