If it is set up incorrectly, this can lead to overrun problems. If you have power management enabled, try disabling it, since this can interfere with modem operations. Ensure that you don't have a resource conflict with the modem or the serial port it is using. Try using a different COM port, if this is an external modem, if possible. There could be a problem related to the port.
Screen savers can sometimes cause overrun problems by kicking in at the wrong time.
Try disabling the screen saver. Block mode causes hard disks to perform larger transfers than they normally would on each interrupt. Block mode is normally controlled by this BIOS setting . Note that disabling this feature can lead to a small decrease in hard disk performance. If you have any automatic virus scanning software running, try disabling it.