Serial Port Communication Under MS-DOS 6.22

Please I have MS-DOS 6.22 that runs on a Pentium 4 3.0 GHz machine with 512MB of ram. I use this machine to run a legacy application. The legacy application uses serial port to communicate with an external hardware. The problem is that the connection is not stable sometimes it fails and so the program fails to communicate and terminate.

I was successful to use Laplink ( a program that uses serial/parallel cables to transfer files) to transfer files using a serial cable and the files were transferred successfully although they were much slower than usual which i suspected that the serial communication is not perfect and that exactly what happens with my legacy application, but in the case of my application it does not retry and it fails if it does not find a reliable communication

The legacy application works perfectly on any machine less than 120MHz, but any other machine greater that 233 MHz the serial communication fails

Please advise
any help would be much appreciated