there's some modem tweaks within windows...i haven't used dialup in the longest time...so i forgot everything..you're going to have to do a google search for those tweaks...there's no downloads nessary..maybe in some tweaks u do...
also, it might also be your ISP..check w/ them and have them ping your modem/phone line..to see everything's ok...also, when you have callwaiting and someone tries to call you, sometimes your internet gets all whacked up...
try putting it in a different phone outlet if that does anything....check the phone line to see if there's no small break in connection..(all it takes is a small tiny laceration in the wire to cause unessary noise within the data transfers...also having huge speakers next to the phone lines screws things up...)
well, hope that helps...