Go to Netzero.com and setup a free account with them. Then test @ PCPitstop through their connection. If the speed is the same, then it is your modem. If it is higher, then it is your ISP.
To add ot the suggestions, you might also try changing the dialup number your ISP has you using. I used to have to do that every now and then when one line connected at sub-par speeds. Your ISP usually has several numbers to choose from unless you live in a rural area.