You appear to be running with the 802.15.4 XBee radios at a 115.2kbps data rate and streaming limit.
There are a few things you need to know about this product and that rate.
First, due to clock divisors at the 115.2kbps data rate, the closest rate that the module is able to achieve is 112kbps or 111,111bps. While this rate is within the nominal rate of 115.2kbps, it is on the lower side of the tolerance. If your devices connected to the radio is on the high side of the tolerance, a baud rate mismatch can occur. To help address that, either open the COM port up at 112kbps or use two stop bits instead of 1. The extra bit slows down the communications just enough to address the tolerance issues. With the information you have provided, I suspect making this adjustment will resolve the issue.
Second, the 802.15.4 XBee have a streaming limit of 80kbps with the ability to support data bursts of up to 115.2kbps. You most likely will need to use hardware flow control to keep from over running the buffers.