help: serial port problem

I am a radio amateur. I have a computer controlled radio that uses the serial port, cable, and a software program designed for this setup. It worked for about a year as per specs. Then, it just stopped working. Much trouble shooting reveals that the serial port is "not working." I have used a USB-serial emulator that works fine. So, the radio, cable and software are okay. Device manager says that the serial port is working. But the software can't contact the radio through it. There has been no change in the software. I build the computer some time ago with an Asus M2N-SLI delux MB. Computer otherwise works as normal in all respects. Any suggestions for trouble shooting much appreciated. Thanks.
...Niky