OK. So don't try ping tests to your ATT gateway listed in your modem web pages (not your home gateway) or use the standard version of PingPlotter (30-day free trial) to that gateway and beyond-- because that could show local or non-local problems. Don't post your signal strength from the modem web pages.
I meant "loaded" as in fully loaded or overloaded... Having seen your test I tried it and got a very low set of numbers. Then I switched sites and got my normal speeds.