XBee S2 stops responding

Hi there. I have a big problem with my XBee modules.
I own 2 radios XBee series 2 and XBee Explorer USB v2 - SparkFun.

A: PC -> XBee Explorer USB -> XBee S2
B: ATmega8 -> XBee S2

Configuration:

A: router (PAN ID: 7777)
baudrate: 9600
flow control: none
data bits: 8
parity: none
stop bits: 1

B: coordinator (PAN ID: 7777)
baudrate: 9600
flow control: none
data bits: 8
parity: none
stop bits: 1
atmega: 4MHz crystal oscilator, UBBR_value = 25

Describe:

A:
I wrote simple code in C (Windows 7) which sends in rotation 2 API frames:
7E 00 11 10 00 00 13 A2 00 40 BD 27 A0 FF FE 00 00 00 03 64 12
7E 00 11 10 00 00 13 A2 00 40 BD 27 A0 FF FE 00 00 00 03 00 76
Between above operations are 5 sec. break (delay).

B:
ATmega receive data with interrupt from XBee correctly and in this case switch LED light like this:
00 03 64 -> turn on the led with id=0x03
00 03 00 -> turn off the led with id=0x03

Everything works fine, led is blinking but after 6 sendings XBee A (conected to XBee Explorer) stops responding.
I suppose that buffer is overloaded. I can’t even get access to this module from X-CTU.
When I unplug and plug XBee A to USB the problem comes back after 6 trials of sending.
I don’t understand this situation, 5 seconds of break is a long time…
What should I do?
Thank you for your help.
Tomas

Try using transparent mode first and eliminate API mode. Next, reduce the time between transmissions or use Hardware flow control and read the serial port after sending the data so you can clear the receive and status packets.