You are looking good.
What do you mean by "first receive data"
Your post processing may be taking a long time and subsequent data is overwriting the 64 character receive buffer.
Check out how many characters are available during each pass. If it is 64, you are probably missing data.
If that is the case, you may need to process one character (as was mentioned earlier) at a time and check for the ',' yourself, the strtok overhead may be too much.