Electrical – Arduino PID temperature controller

I am running the below code on an Arduino Uno and it is partially working.
I will try to explain the problem step by step:

1. I start the soldering station from AC 230V
2. The soldering station heats up to maximum (I measure 18mV on the TC at maximum)
3. I reduce voluntary the temperature by rotating the pot to the minimum
4. The temperature (and the voltage on the TC) starts to decrease
5. When the voltage on the TC is about 16-17mV then the power to the soldering iron starts to increase suddenly. And I cannot do anything to make it stable or to decrease it.
6. The voltage on the TC continues to rise, and when it reaches 22-23mV, I unplug the soldering station from the wall socket, to protect the heater of the soldering iron.

I tried the following solutions for my problem: I checked individually each part of the schematic and all of them were good.
I found that this event happens, when in the code the PID error is equal (=) to set point. This makes the PID value to be 7400 and in the delayMicroseconds(maximum_firing_delay - PID_value); line of code the result is zero (0), making the entire sine wave to be present on the soldering iron heater. I measured those things in serial monitor.I have also monitored the voltage on the TC and on the output of the OpAmp and it seems to be ok, with no spikes or suddenly increases or decreases.

Schematic: https://ibb.co/hKn5kjL

//Inputs and outputs
int firing_pin = 5;
int zero_cross = 2;

//Variables
int last_CH1_state = 0;
bool zero_cross_detected = false;
int firing_delay = 7400;

//////////////////////////////////////////////////////
int maximum_firing_delay = 7400;
/*Later in the code you will se that the maximum delay after the zero detection
 * is 7400. Why? Well, we know that the 220V AC voltage has a frequency of around 50-60HZ so
 * the period is between 20ms and 16ms, depending on the country. We control the firing
 * delay each half period so each 10ms or 8 ms. To amke sure we wont pass thsoe 10ms, I've made tests
 * and the 7400us or 7.4ms was a good value. Measure your frequency and chande that value later */
//////////////////////////////////////////////////////

unsigned long previousMillis = 0; 
unsigned long currentMillis = 0;
int temp_read_Delay = 500;
int real_temperature = 0;
int setpoint = 100;

//PID variables
float PID_error = 0;
float previous_error = 0;
float elapsedTime, Time, timePrev;
int PID_value = 0;
//PID constants
int kp = 30;   int ki= 10;   int kd = 15;
int PID_p = 0;    int PID_i = 0;    int PID_d = 0;


void setup() {
  //Define the pins
  Serial.begin(9600);
  pinMode (firing_pin,OUTPUT); 
  pinMode (zero_cross,INPUT);   
  PCICR |= (1 << PCIE2);    //enable scan                                                 
  PCMSK2 |= (1 << PCINT18);  //Set pin D2 (zero cross input) trigger an interrupt on state change.
}


void loop() {    
  currentMillis = millis();           //Save the value of time before the loop
  if(currentMillis - previousMillis >= temp_read_Delay){
    previousMillis += temp_read_Delay;              //Increase the previous time for next loop
    //get the real temperature in Celsius degrees
    // added by Mike
    for(int i=0;i<50;i++)
    real_temperature += analogRead(A0);  
    real_temperature /= 50;
    real_temperature = map(real_temperature, 0, 550, 25, 400);
    // end added by Mike
    setpoint = analogRead(A1);
    setpoint = map(setpoint, 0, 1023, 150, 400);
    PID_error = setpoint - real_temperature;        //Calculate the pid ERROR

    if(PID_error > 30)                              //integral constant will only affect errors below 30ºC             
    {PID_i = 0;}

    PID_p = kp * PID_error;                         //Calculate the P value
    PID_i = PID_i + (ki * PID_error);               //Calculate the I value
    timePrev = Time;                    // the previous time is stored before the actual time read
    Time = millis();                    // actual time read
    elapsedTime = (Time - timePrev) / 1000;   
    PID_d = kd*((PID_error - previous_error)/elapsedTime);  //Calculate the D value
    PID_value = PID_p + PID_i + PID_d;                      //Calculate total PID value

    //We define firing delay range between 0 and 7400. Read above why 7400!!!!!!!
    if(PID_value < 0) // initial it was <
    {      PID_value = 0;       }
    if(PID_value > 7400) // initial it was >
    {      PID_value = 7400;    }
    previous_error = PID_error; //Remember to store the previous error.
//    Serial.println("PID_error=");
//    Serial.println(PID_error);
//    Serial.println("real_temperature=");
//    Serial.println(real_temperature);
//    Serial.println("setpoint=");
//    Serial.println(setpoint);
//    Serial.println("PID_value=");
//    Serial.println(PID_value);
  }

  //If the zero cross interruption was detected we create the 100us firing pulse  
  if (zero_cross_detected)     
    {
      delayMicroseconds(maximum_firing_delay - PID_value); //This delay controls the power
      digitalWrite(firing_pin,HIGH);
      delayMicroseconds(100);
      digitalWrite(firing_pin,LOW);
      zero_cross_detected = false;
    } 
}

//This is the interruption routine (pind D8(zero cross), D11(increase) and D12(decrease))
//----------------------------------------------

ISR(PCINT2_vect){
  ///////////////////////////////////////Input from optocoupler
  if(PIND & B00000100){            //We make an AND with the state register, We verify if pin D2 is HIGH???
    if(last_CH1_state == 0){       //If the last state was 0, then we have a state change...
      zero_cross_detected = true;  //We have detected a state change! We need both falling and rising edges
    }
  }
  else if(last_CH1_state == 1){    //If pin 2 is LOW and the last state was HIGH then we have a state change      
    zero_cross_detected = true;    //We haev detected a state change!  We need both falling and rising edges.
    last_CH1_state = 0;            //Store the current state into the last state for the next loop
    }
}