If you are using a high speed modem (14,400 Baud or above), your serial ports needs to be capable of handling the throughput that such a modem is capable of producing. This requires your serial port to use a modern UART (Universal Asynchronous Receiver Transmitter) such as a 16550(A). If you are using an old machine (or old serial card), it is quite possible that your serial port has only an 8250 UART, which will cause you considerable problems when used with a high speed modem.
Use the command setserial -a /dev/ttySx
to get Linux to report to you the type of UART you have. If you
do not have a 16550 type UART, invest in a new serial card (available
for under $50).