Help diagnose this Uverse problem, por favor

The SN margin on my line drops to barely within acceptable parameters from early to late afternoon, resulting in multiple disconnects. Why always this time of day? I guess there could be local interference of some kind, but it's a residential area and there's no obvious source that I can think of or have noticed.

My other possibility is something to do with sunlight or the heat of the day. This wasn't happening during winter and spring, and my connection improved last week during a thunderstorm that dropped the temperature about 20 degrees.

Uverse was unusable from around noon today to about 6 PM, now it's fine. Any ideas? ATT has been no help.