Centurylink Internet — slower than what i'm paying for

I have been paying for 3Mbps service for over two years. For much of that time, the service has delivered between .7 and 1.3 Mbps as verified by both speedtest.net and speakeasy.com. I have called the Centurylink customer support several times, and each time they want to run me through hours, literally hours, of diagnostics on my computer to try and prove it's my problem. They say, unless I submit to this ridiculousness each time I call, then they are not authorized to forward the problem to engineering. Thing is, I've got two computers, one directly wired and the other on a wireless router, and they both perform at similar speeds. The problem is clearly related to bandwidth exhaust somewhere down the line, but Centurylink insists on running me through their "flow chart" each time I've called about it. Maddening.
Wouldn't be such a problem if their slogan was "Well do the best we can, " but it's insulting when they advertise "Your connection to our network is consistently fast, all day, every day." That is simply a lie.