I seem to top off at around 600 mbps then drop drastically wondering why this would be happening
I've had the same service from AT&T for 2 years. So I suggest to run a speed test from Speedtest.net and select the nearest AT&T server. Also run a speedtest directly from the AT&T website. AT&T wont do shit if you at least get 500mbps down and 500mbps up. Check this out and goto the bottom [ Register or Signin to view external links. ]
Also make sure your hardware can handle the gigabit throughput. Check your CPU load when running a speedtest to make sure there aren't issues.
Are you trying to reduce bufferbloat for gaming? I also use DSLReports, great website.