The 16550 chip, on the other hand, is reliable up to 115200. Windows 3.1 users can check to see if you have an older 8250 or 16450 serial chip in the MSD program. If so, you'll want to get a new serial card, one that supports the higher speeds. If you don't have the MSD program, check with your computer's manufacturer about the type of serial chip used in your machine. If you have an internal modem, you don't need to worry about the uart, or kind of serial chip your computer has.
Internal modems come with their own high-speed serial chips.
The most common reason is discussed in the answer above. If you're using Windows 3.1, you might want to check a few settings in your system configuration file. One of the windows that opens will be system.ini. This is the one we want. Some video drivers built around the s3 video chip also hold CPU cycles and therefore cause overruns as well.