I had that same problem back when I used dialup. The modem would work okay for a while and then just start creeping until it was useless. I thought it was everything but the modem. Guess what... it was the modem. I put a new 56k in it and realized I had forgotten just how fast it used to be. Just because it connects doesn't mean something on the modem card isn't overheating or changing value. I recommend trying a new one. It worked for me.
But before you rush out to the store, pick up the phone and push a button to kill the dial tone and listen. Is it quiet with no buzz or crackling? Sometimes a cordless phone charger/pwr supply can induce a buzz that will trash your data stream. So can an old flourescent light fixture. If you hear any kind of noise, unplug all the other phones in the house (and their power supplies if they have them) and try again with an old cord phone. Then you can go outside to the box, open it and unplug the line to the house, then plug a phone into the test jack. Push a button to kill the dial tone. Is it quiet there? If it is, then the feed from their end is good. If it sounds clear, it's usually good enough for data... unless you're deaf.
Water in your phone box outside will corrode the wires and cause static and crackling. That's worth a check.
Now I'm running broadband, but I did have to eliminate some other monthly bills to squeeze it in, so I know where you're coming from.
Let us know how it works out. Maybe the store will agree to take the modem back If it turns out not to be the problem.
Edited by RknRusty, 14 March 2009 - 10:43 PM.