If I'm reading your post correctly, you are saying that transmitting data from your laptop, through your Xstream, you are losing bytes of data occasionally. Correct? Also, does that mean you have an Xstream radio attached to your microcontroller too?
It might help if you post your configuration file for the Xstream radios.
What's the environment like? what kind of signal strength do each of them get? are they line of sight? If you put them close together in a lab, does the problem go away?