If you have LocalTalk active, it can interfere with other high-speed serial activity. Modem connections with DTE or port speeds of 19,200 and above can be particularly sensitive to this issue.

If you're using LocalTalk, consider turning AppleTalk off (using the Chooser) while you're doing modem transfers. Keeping your DTE or port speed the same as your modem connection can help. Using hardware handshaking can also help. Consult the documentation for your modem and communication software for more information about DTE speed and hardware handshaking.