After 2 modems and a new computer I was still getting DL speeds of 300kbps when I had subscribed for 3.0M
Well, after pulling my hair out for a month, they finally sent another line tech to my house, who within 2 minutes says to me, "I know exactly what the problem is". He called the central office and it was as simple as switching me to a different router there because evidently overloads can exist.
10 seconds later my DL speeds were 2885kbps.
Thought I'd share this as a possiblity that when all else fails it might really actually be the ISPs fault.