Digi Xbee Pro S2C - Transparent Mode Throughput

I am running a test with a single endpoint connected to my co-coordinator. For a test I am sending very small packets of serial data (only 2 bytes). This results in an ‘Explicit RX Indicator’ packet of 20 bytes. I seem to be reaching some limit that I am unaware of. If I transmit my serial data at a rate at 10Hz (every 100ms) then the RF packets received by the coordinator ‘in sync’ with my transmitted data (obviously lagging the transmitted data by a few ms). I can see this on an oscilloscope. The problem is if I increase the data rate above 10Hz, the data then moves out of sync and I then start to get packets containing more than one serial transmission.

Is there some maximum limit on throughput in transparent mode. I have the ‘Packetization Parameter’(RO) set to 3, so at 115200 baud rate, the packet should be transmitted from the endpoint within approximately 0.26ms after receiving the last character of serial data. So sending data at a rate faster than 10HZ should not be an issue.

Any help would be appreciated