I suggest finding the IP address of your ATT gateway. You can find that in the Settings>Broadband>Status. Look for the item called Default Gateway. Write down that IP address.
Then ping that over long periods. I suggest the standard version of Ping Plotter. It has a 30-day free trial. It will graph your connectivity vs time, and that can help describe what you are seeing. If there is a time pattern, you could spot that too.
There are other ping programs that you might like.