What you should do - is call At&T and ask them with the plan you have, what are the speeds you should be seeing.... don't let them ask you what speeds you are getting first.... and to be honest, their lowest plan sucks balls, - their highest plan is pretty darn good if your close to a fiber optic line.... but I believe there is a middle ground plan...
Then after they tell you what speeds you are supposed to get with your plan compare those numbers to what you are truly getting....
Heres the thing, the internet and ISP's are running out of bandwidth until they get full coverage with Fiber Optic nation wide backbone setup, and I'm pretty confident that they put restrictions on lines that don't use alot of bandwidth on a normal basis, so they can dedicate that usage across the board - You won't really even notice it unless you are a serious high bandwidth user - even though you are paying for premium connection.
Now I'd be willing to wager that after you make that phone call that all of a sudden your bandwidth jumps up leaps and bounds from what you are seeing right now...
I have Road Runner turbo with Time Warner Cable - I only use them for internet, and just recently I went through this same thing, speed testing from various sites on the net.... gather up the speeds I'm showing and then make a call to Time Warner..... I ask, they tell, and then low and behold - test again and HEY its all better now... They checked my line and clicked a button and imagine my surprise - my bandwidth is above what it should be and then it settled down to what is stated to be delivered...
Personally I have an idea that they have a computer that starts regulating how much bandwidth is dispersed and where they can cut corners which can be reset straight with the click of a button, and they utilize this to help their own internal resources, whilest still making that top dollar...
Also, keep in mind, the ISP likes to have you check while you are on the phone with them certain bandwidth sites that always give back the best numbers like fiber optic backbones to certain cities from them.... so things always look pristine on their behalf.... - oh and thats usually after they click that button....
Oh and also - you can't rule out the possibility that if your modem is old it might be failing and giving you really poor results as well, in which a CS guy should be dispatched to test and replace if necessary. AT&T's modems suck, and get replaced all the time - trust me.... I went through 5 of them at work over a period of 3-4 years. GRRR I hate AT&T.....
