Arduino code not starting at beginning after poweroff

Question

I created a few Arduino-XBee temperature sensor nodes that send data to another XBee node.js server. I'm running into a few issues trying to write a function to initiate a handshake and receive an identifier number to start sending data (so its scalable rather than hard-coded on each) while the receiver is already communicating with pre-established nodes.

The first issue I'm having is that the Arduino nodes do not seem to start from the beginning of their code when I unplug them and then reapply power. They already think they have their identifier number if another node is running, and I have to push the reset button on them to get them to run their code fresh and establish their own handshake ID. Is there anything I can put in the code or a way that I can easily modify the circuitry to make them actually reset and start fresh when they lose power?

The circuitry I'm using is a basic thermistor voltage-divider, XBee module, and 5V battery pack. Here's what I've got so far for the code on the Arduino side:

#include <SoftwareSerial.h>
#include <math.h>

// Global variables
int AnalogPin = A2; // has much less jumpy readings than A0 (maybe A0 slightly damage from overuse)
float V_source = 4.90; // measured
SoftwareSerial XBee(2,3); // RX, TX
String identifier = "";
int receivedID = 0;

// Calculate thermistor voltage
float Voltage(float RawADC) {
  float meter_calibration = 0; // don't seem to need, matches my multimeter exactly
  float V_therm = (V_source * RawADC) / 1024.0;
  V_therm += meter_calibration;
  return V_therm;
}

// Calculate thermistor resistance
// sometimes a bit off what voltage divider eq says it should be (probably due to input impedance of ADC)
float Thermistor(float V_therm) {
  float resistance_calibration = 25; // possibly small breadboard/wires resistance
  float R_known = 560 + resistance_calibration; // measured (1% tol resistor)
  float R_therm =  ((R_known * V_therm) / (V_source - V_therm));
  return R_therm;
}

// Base conversion
float log2(float num) {
  return (log(num) / log(2));
}

// Calculate temperature
float Temperature(float R_therm) {
  float A = 0.0164872;
  float B = -0.00158538;
  float C = 0.0000033813;
  float temp_K = 1 / (A + B*log2(R_therm) + C*pow(log2(R_therm),3));
  return temp_K;
}

// Convert to Celsius or Fahrenheit
float ConvertTemp(float temp_K, char T) {
  if (T == 'C')  
    return (temp_K - 273.15);
  else if (T == 'F') 
    return (temp_K * 1.8 - 459.67);
}



// Setup
void setup() {
  int idStart = 0;
  receivedID = 0;
  Serial.begin(9600);
  pinMode(AnalogPin, INPUT);
  XBee.begin(9600);
  for (int i=0; i<100; i++) {
    Serial.print("\n");
  }
  Serial.println("Broadcasting...");
}

// Main
void loop() {
  delay(1000);

  // Initiate handshake
  if (identifier == "") {
      XBee.print("BROADCASTING"); // ascii text
      XBee.write("\n");  // binary data
  }
  delay(3000);



  // Receive buffer, check for and confirm ID
  if (XBee.available() && (identifier == "")) { 
    String msg = "";
    char r = XBee.read();

/*  
    // Error checking for first-only numeric ID (testing)
    msg = msg + r;
    String intString = "";
    int intStart = 0;
    int i = 0;
    while (intStart != 2 && i < msg.length()) {
      if (intStart == 0) {
        if (isDigit(msg[i])) {
              intString += msg[i];
              intStart = 1;
           }
      }
      else if (intStart == 1) {
        if (isDigit(msg[i])) {
              intString += msg[i];
              intStart = 1;
           }
      }
      else if (!isDigit(msg[i])) {
        intStart = 2;
      }
      i++; 
    } 
    Serial.print("testing: ");
    Serial.println(intString);
    identifier = intString;
*/

    identifier = r;
  }

  // handshake established
  if ((identifier != "") && (receivedID == 0)) {
    Serial.println();
    Serial.print("RECEIVED IDENTIFIER: ");
    Serial.println(identifier);
    Serial.println();
    receivedID = 1;
  }




  // Calculate and send temperature along with identifier
  while (receivedID == 1) {
        // ADC and V_therm values
    /* Good to get two or more analog readings and discard the first or take the average
    apparently analog pins are multiplexed so this gives it time to settle */
    float RawADC_1 = analogRead(AnalogPin); // toss 1
    float RawADC_2 = analogRead(AnalogPin); // average 2,3
    float RawADC_3 = analogRead(AnalogPin);
    float RawADC_avg = (RawADC_2 + RawADC_3) / 2.00;
    Serial.print("Raw ADC value: ");
    Serial.println(RawADC_avg);
    float V_therm = Voltage(RawADC_avg);
    Serial.print("Voltage across thermistor: ");
    Serial.println(V_therm);

    // Thermistor resistance
    float R_therm = Thermistor(V_therm);    
    Serial.print("Resistance of thermistor: ");
    Serial.println(R_therm);

    // Temperature in Kelvin, Celsius, Fahrenheit
    float temp_K = Temperature(R_therm);
    Serial.print(temp_K);
    Serial.print("K   ");
    float temp_C = ConvertTemp(temp_K, 'C');
    float temp_F = ConvertTemp(temp_K, 'F');
    Serial.print(temp_C);
    Serial.print("C   ");
    Serial.print(temp_F);
    Serial.println("F   ");

    // XBee
    String mytemp = String(temp_F);
    String msg = String(identifier + " " + mytemp);  
    XBee.print(msg); // ascii text
    XBee.write("\n");  // binary data
    Serial.print("Message sent to XBee: ");
    Serial.println(msg);
    Serial.println();


    delay(2000);
  }


}