If you have bad phone lines that can definitely contribute to speed issues. I was having the same thing and Qwest got it taken care of.

Go into your modem WAN status and see what the following values say:
Number of Retrains
Number of Retrains Elapsed Time
Near End CRC Errors (I/F)
Far End CRC Errors (I/F)
Near End CRC(Within last 30 mins)(I/F)
Far End CRC(Within last 30 mins)(I/F)
Near End RS FEC (I/F)
Far End RS FEC (I/F)
Near End FEC(Within last 30 mins)(I/F)
Far End FEC(Within last 30 mins)(I/F)
Discarded Packets(Within last 30 mins)
SNR Margin (Downstream/Upstream)
Attenuation (Downstream/Upstream)

If you don't have some of those that's fine, just paste up your modem stats that you have.