You could cancel the AT&T service, plan on waiting at least one year, and then start fresh with AT&T. After cancellation, you could type a letter explaining why you cancelled and mail it to AT&T. I know it may seem drastic, but as long as AT&T has you as an internet customer who is paying them something decent, they may not take the issue seriously. A cancellation followed up with a written letter explaining why you cancelled may get their attention in such a way as to generate purposeful action leading to better service.
Depending on where you live, you might consider using something like
Millenicom »
millenicom.com/plans/#ho ··· pot-planWireleesNWiFi »
www.wirelessnwifi.com/Hi ··· -ServiceNot low cost, and it has strict usage caps, but if you are in a decent cellular service zone, you could get better data transfer rates.
I assume there are no other ISPs such as a cable provider in your area or you would have already cancelled and switched to them, at least on a trial basis.