Arduino Uno and 7 segment display - wrong value displayed

Question

I am working at this project and I found that if I have set the temperature by rotating the pot to 312 C, the iron started heating up, but when it was at about 60 C then on the display (7 segment, 3 digit) appeared 312 (the temperature that I have set) and then appeared about 70 C. I did not rotate the pot, the 312 appeared from "nowhere". I powered on again the soldering station and it showed 35 C - then 312 C - and then 52 C and then the temperature rise normally. I set the pot randomly at 244 C and the problem did not appeared. The problem also appeared at 344 C set temperature. I tried with different soldering irons and 2 different potentiometers and the problem still persist. I have also tried with different Arduino boards, but the problem still persist.

Is there a solution for this problem ?

The project page: https://www.allaboutcircuits.com/projects/do-it-yourself-soldering-station-with-an-atmega8/?utm_source=eetech&utm_medium=eetech-social&utm_campaign=reposts-projects/

Explanation of the schematic: J2 from "schematic.jpg" is connected to J1 of "7 segment.jpg", pin to pin. The last 4 pins of J2 from "schematic.jpg" are connected to J3 from "7 segment.jpg" and the last pin from J2 ("schematic.jpg") is connected to J6 from the "7 segment.jpg".

https://ibb.co/Zdrt1Hq https://ibb.co/DC0LzV2

My code:

//Thank you Alex from https://geektimes.ru/ for help with led array function
//AllAboutCircuits.com
//epilepsynerd.wordpress.com

#include <PID_v1.h>

//This array contains what segments need to be turned on to display numbers 0-9
byte const digits[] = {
  B00111111, B00000110, B01011011, B01001111, B01100110, B01101101, B01111101, B00000111, B01111111, B01101111
};

int digit_common_pins[] = {8, 9, 10}; //Common pins for the triple 7-Segment LED display
int max_digits = 3;
int current_digit = max_digits - 1;

unsigned long updaterate = 1200; //Change how fast the display updates. No lower than 500
unsigned long lastupdate;

int temperature = 0;

//Define Variables we'll be connecting to
double Setpoint, Input, Output;

//Define the aggressive and conservative Tuning Parameters
double aggKp = 4, aggKi = 0.2, aggKd = 1;
double consKp = 1, consKi = 0.05, consKd = 0.25;

//Specify the links and initial tuning parameters
PID myPID(&Input, &Output, &Setpoint, consKp, consKi, consKd, DIRECT);

void setup()
{
  DDRD = B11111111;  // sets Arduino pins 0 to 7 as outputs
  for (int y = 0; y < max_digits; y++)
  {
    pinMode(digit_common_pins[y], OUTPUT);
  }
  //We do not want to drive the soldering iron at 100% because it may burn, so we set it to about 85% (220/255)
  myPID.SetOutputLimits(0, 220);
  myPID.SetMode(AUTOMATIC);
  lastupdate = millis();
  Setpoint = 0;
}


void loop() {
  //Read temperature
  Input = 0;
  for(int i=0;i<50;i++)
  Input += analogRead(A0);
  Input /= 50;
  //Transform the 10bit reading into degrees celsius
  Input = map(Input, 0, 550, 25, 400);
  //Display temperature
  if (millis() - lastupdate > updaterate) {
    lastupdate = millis();
    temperature = Input;
  }
  //Read setpoint and transform it into degrees celsius(minimum 150, maximum 350)
  double newSetpoint = analogRead(A1);
  newSetpoint = map(newSetpoint, 0, 1023, 150, 400);
  //Display setpoint
  if (abs(newSetpoint - Setpoint) > 3) {
    Setpoint = newSetpoint;
    temperature = newSetpoint;
    lastupdate = millis();
  }

  double gap = abs(Setpoint - Input); //distance away from setpoint

  if (gap < 10)
  { //we're close to setpoint, use conservative tuning parameters
    myPID.SetTunings(consKp, consKi, consKd);
  }
  else
  {
    //we're far from setpoint, use aggressive tuning parameters
    myPID.SetTunings(aggKp, aggKi, aggKd);
  }

  myPID.Compute();
  //Drive the output
  analogWrite(11, Output);
  //Display the temperature
  show(temperature);
}

void show(int value) {
  int digits_array[] = {};
  boolean empty_most_significant = true;
  for (int z = max_digits - 1; z >= 0; z--) //Cycle through each digit
  {
    digits_array[z] = value / pow(10, z); //We now take each digit from the number
    if (digits_array[z] != 0 ) empty_most_significant = false; //Do not display leading zeros
    value = value - digits_array[z] * pow(10, z);
    if (z == current_digit)
    {
      if (!empty_most_significant || z == 0) { //Check to see that we do not have leading zeros and display the current digit
        PORTD = ~digits[digits_array[z]]; //Remove ~ for common cathode
      }
      else
      {
        PORTD = B11111111;
      }
      digitalWrite(digit_common_pins[z], LOW);//Change to LOW for common cathode
    } else {
      digitalWrite(digit_common_pins[z], HIGH);//Change to HIGH for common cathode
    }

  }
  current_digit--;
  if (current_digit < 0)
  {
    current_digit = max_digits; //Start over
  }
}```