There's nothing wrong with my modem, I always get connections of 57,600 or 115,200.

There could still be a problem with your modem or with your modem drivers if you are seeing a connection of 57600, 115200, or even 38400.  Follow this link for an explanation.

Link to other information  U.S. Robotics Sportster 28.8 Modem Loses Connection Speed