Assuming its not wifi related (Test with a wired connection)
Firs thing to check is that usage within your own premises inst increasing, but if its very gradual this is unlikely.
The above being the case, the most likely cause is congestion on AT&T network between your local exchange and the core. Most commonly you'll see that on older or more remote exchanges with a wireless backhaul. AT&T L1 staff may not be allowed to inform you of this, so instead fob you off with "its all fine here".
To get some data to push the issue I would suggest you run tracert to goole.com say, find out the first hop after your router and then run PingPlotter against it. This will be testing within AT&Ts network and produce a nice graph of the effects of congestion. Then ask them when it will be upgraded, and if they won't cancel your service, its the only way to get these things moved along.