with such short times between each byte, reading the data one byte at a time is not a good idea.
you certainly know that Windows is a multitasking OS, which means that many processes are running in parallel on the computer. since you have less processors than you have processes running, the system is cheating by executing small amount of each process regularly, which gives the impression of parallel execution. each execution frame is in the order of the millisecond, which means that your process may sometimes not be running for some millisecond. it is for the same reason that timers on windows are limited to a 5ms resolutions (now, really we can have a better resolution by using some specialized calls). so, you cannot guarantee that a byte you read did really come less than a ms after the previous byte. (sometimes, windows decides to swap a lot of stuff from the pagefile, locking the system for only half a second, but that's 500 characters that should have been read by the time windows schedules your process again).
you should also take into account the input buffer on the serial port: once the comm port is open and configured, every bytes coming in are stored into a buffer under the hood, and using ComRdByte reads from the buffer. the timeout comes into play only when the buffer is empty. this way, you can also not guarantee that a byte you read did really come less than a ms after the previous byte.
you should really consider reworking your reading process to read strings of multiple bytes at once: it will be much easier to program, less prone to timeout errors, and way more robust.
