variable latency and hanging on of xbee pro 2

I’m new to this forum.
I’m using two Xbee pro serie 2 (XBP24-BWIT-004 revD).
I’m having two of them at this time but I will have 6 at the end of the project.
I have configure them with x-ctu and a sprakfun xbee usb explorer (
Here is the screen capture of the settings of the coordinator

and the screen capture of the router/end device

Actually the communication works but there is strange behaviour : there is a very variable latency, and sometimes the network seems to be broken for a while (the rss led on the sparkfun explorer is not green). I have check on the router device that the ATSM returns 0. So the xbee is on a “no sleep mode”.
I’m using the xbee modules with an arduino with this code

int x = 0;                              // a place to hold pin values
int ledpin = 13;

void setup()
  digitalWrite(13,HIGH);              ///startup blink

void loop()
if (Serial.available() > 0){         // Check serial buffer for characters
    if ( == 'r') {       // If an 'r' is received then read the pins
     x = analogRead(0);                //read the value on analog 0
    sendValue (x);
    Serial.println();                 // Send a carriage returnt to mark end of pin data. 
    delay (100);                        // add a delay to prevent crashing/overloading of the serial port

void sendValue (int x){              // function to send the pin value followed by a "space". 
 Serial.print(32, BYTE); 

Here is a very schematic representation of my setup

On one side I have this arduino with this program connected with the router xbee. It’s replying to ‘r’ message and sends the value of the analog input 0 (I have attached a potentiometer to it). On the other side I have an xbee (the corrdinator) on an usb explorer connected to the computer. I’m using a Max program on the computer that sends ‘r’ message to the serial port.
It works with a very variable latency (from 500 ms to 3 seconds) and
sometimes I see that the router seems to sleep (the rss led is not green).
Is it the correct configuration for the xbee ?
I didn’t manage to get the api mode work on it and I thought that AT mode was a correct way to go for that kind of network.
Any advice will be appreciated