Xbee [Unusual Behavior in Pin-Hibernate]

I have interfaced Arduino Uno with Xbee-Series 2 module.
Both are perfectly working till when i didn't put the xbee in sleep.

When i Put Xbee in sleep Sometime it send data, sometimes it don't
and sometimes i got previous data. Which i already recieve.

I am using Xbee in transparent mode. Is there any way by which i can clear the buffer of xbee [in At Mode].

Waiting for your Reply

Thanks in Advance..