Sensor lines flashing

Hi guys. I’ve finally got my project to a point where the DS3231 RTC prints the time date on the first 2 lines of the 20x4 LCD and the DHT11 prints temp and humidity on the 3rd and 4th and switches a fan via relay.
Something though is conflicting and after about 20 seconds temp and humidity start blinking at 2 second intervals. Any ideas as to what is causing this

Thanks
Chris

#include <DHT.h>
#include <RTClib.h>
#include <Wire.h>
#include "ds3231.h"
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 20, 4);

#define BUFF_MAX 128  // rtc
#define DHTPIN 2     // what pin we're connected to
#define DHTTYPE DHT11  // DHT 11  
#define fan 4     // Pin the relay is connectd to

int maxHum = 100;  // max humidity
int maxTemp = 31;  // max temperature

DHT dht(DHTPIN, DHTTYPE);

uint8_t time[8];
char recv[BUFF_MAX];
unsigned int recv_size = 0;
unsigned long prev, interval = 1000;

void setup()
{
  Serial.begin(9600);
  pinMode(fan, OUTPUT);
  dht.begin();
  Wire.begin();
  lcd.backlight();
  DS3231_init(DS3231_CONTROL_INTCN);
  memset(recv, 0, BUFF_MAX);
  Serial.println("GET time");
  lcd.begin(20, 4);
  lcd.clear();
  //Serial.println("Setting time");
  //parse_cmd("T003521229022020",16);
  //           TssmmhhWDDMMYYYY
}

void loop() {

  float h = dht.readHumidity();
  float c = dht.readTemperature();

  lcd.setCursor(0, 2);

  lcd.print("Temperature: ");
  lcd.print(c);
  lcd.print((char)223);
  lcd.print("C");

  lcd.setCursor(2, 3);
  lcd.print("Humidity: ");
  lcd.print(h);
  lcd.print("%");
  //delay(1000);
  //lcd.clear();
  char in;
  char tempF[6];
  float temperature;
  char buff[BUFF_MAX];
  unsigned long now = millis();
  struct ts t;

  // show time once in a while
  if ((now - prev > interval) && (Serial.available() <= 0)) {
    DS3231_get(&t); //Get time
    parse_cmd("C", 1);
    // temperature = DS3231_get_treg(); //Get temperature
    //dtostrf(temperature, 5, 1, tempF);

    lcd.clear();
    lcd.setCursor(2, 0); // First Line

    lcd.print(t.mday);

    printMonth(t.mon);

    lcd.print(t.year);

    lcd.setCursor(6, 1); // Second Line
    lcd.print(t.hour);
    lcd.print(":");
    if (t.min < 10)
    {
      lcd.print("0");
    }
    lcd.print(t.min);
    lcd.print(":");
    if (t.sec < 10)


    {
      lcd.print("0");
    }
    lcd.print(t.sec);

    float h = dht.readHumidity();
    // Read temperature as Celsius
    float t = dht.readTemperature();

    // Check if any reads failed and exit early (to try again).
    if (isnan(h) || isnan(t)) {
      Serial.println("Failed to read from DHT sensor!");
      return;
    }

    if (h > maxHum || t > maxTemp) {
      digitalWrite(fan, HIGH);
    } else {
      digitalWrite(fan, LOW);
    }

    prev = now;
  }


  if (Serial.available() > 0) {
    in = Serial.read();

    if ((in == 10 || in == 13) && (recv_size > 0)) {
      parse_cmd(recv, recv_size);
      recv_size = 0;
      recv[0] = 0;
    } else if (in < 48 || in > 122) {
      ;       // ignore ~[0-9A-Za-z]
    } else if (recv_size > BUFF_MAX - 2) {   // drop lines that are too long
      // drop
      recv_size = 0;
      recv[0] = 0;
    } else if (recv_size < BUFF_MAX - 2) {
      recv[recv_size] = in;
      recv[recv_size + 1] = 0;
      recv_size += 1;
    }

  }
}

void parse_cmd(char *cmd, int cmdsize)
{
  uint8_t i;
  uint8_t reg_val;
  char buff[BUFF_MAX];
  struct ts t;

  //snprintf(buff, BUFF_MAX, "cmd was '%s' %d\n", cmd, cmdsize);
  //Serial.print(buff);

  // TssmmhhWDDMMYYYY aka set time
  if (cmd[0] == 84 && cmdsize == 16) {
    //T355720619112011
    t.sec = inp2toi(cmd, 1);
    t.min = inp2toi(cmd, 3);
    t.hour = inp2toi(cmd, 5);
    t.wday = inp2toi(cmd, 7);
    t.mday = inp2toi(cmd, 8);
    t.mon = inp2toi(cmd, 10);
    t.year = inp2toi(cmd, 12) * 100 + inp2toi(cmd, 14);
    DS3231_set(t);
    Serial.println("OK");
  } else if (cmd[0] == 49 && cmdsize == 1) {  // "1" get alarm 1
    DS3231_get_a1(&buff[0], 59);
    Serial.println(buff);
  } else if (cmd[0] == 50 && cmdsize == 1) {  // "2" get alarm 1
    DS3231_get_a2(&buff[0], 59);
    Serial.println(buff);
  } else if (cmd[0] == 51 && cmdsize == 1) {  // "3" get aging register
    Serial.print("aging reg is ");
    Serial.println(DS3231_get_aging(), DEC);
  } else if (cmd[0] == 65 && cmdsize == 9) {  // "A" set alarm 1
    DS3231_set_creg(DS3231_CONTROL_INTCN | DS3231_CONTROL_A1IE);
    //ASSMMHHDD
    for (i = 0; i < 4; i++) {
      time[i] = (cmd[2 * i + 1] - 48) * 10 + cmd[2 * i + 2] - 48; // ss, mm, hh, dd
    }
    byte flags[5] = { 0, 0, 0, 0, 0 };
    DS3231_set_a1(time[0], time[1], time[2], time[3], flags);
    DS3231_get_a1(&buff[0], 59);
    Serial.println(buff);
  } else if (cmd[0] == 66 && cmdsize == 7) {  // "B" Set Alarm 2
    DS3231_set_creg(DS3231_CONTROL_INTCN | DS3231_CONTROL_A2IE);
    //BMMHHDD
    for (i = 0; i < 4; i++) {
      time[i] = (cmd[2 * i + 1] - 48) * 10 + cmd[2 * i + 2] - 48; // mm, hh, dd
    }
    byte flags[5] = { 0, 0, 0, 0 };
    DS3231_set_a2(time[0], time[1], time[2], flags);
    DS3231_get_a2(&buff[0], 59);
    Serial.println(buff);
  } else if (cmd[0] == 67 && cmdsize == 1) {  // "C" - get temperature register
    Serial.print("temperature reg is ");
    Serial.println(DS3231_get_treg(), DEC);
  } else if (cmd[0] == 68 && cmdsize == 1) {  // "D" - reset status register alarm flags
    reg_val = DS3231_get_sreg();
    reg_val &= B11111100;
    DS3231_set_sreg(reg_val);
  } else if (cmd[0] == 70 && cmdsize == 1) {  // "F" - custom fct
    reg_val = DS3231_get_addr(0x5);
    Serial.print("orig ");
    Serial.print(reg_val, DEC);
    Serial.print("month is ");
    Serial.println(bcdtodec(reg_val & 0x1F), DEC);
  } else if (cmd[0] == 71 && cmdsize == 1) {  // "G" - set aging status register
    DS3231_set_aging(0);
  } else if (cmd[0] == 83 && cmdsize == 1) {  // "S" - get status register
    Serial.print("status reg is ");
    Serial.println(DS3231_get_sreg(), DEC);
  } else {
    Serial.print("unknown command prefix ");
    Serial.println(cmd[0]);
    Serial.println(cmd[0], DEC);
  }
}

void printMonth(int month)
{
  switch (month)
  {
    case 1: lcd.print(" January "); break;
    case 2: lcd.print(" February "); break;
    case 3: lcd.print("  March "); break;
    case 4: lcd.print("  April "); break;
    case 5: lcd.print("  May "); break;
    case 6: lcd.print("  June "); break;
    case 7: lcd.print("  July "); break;
    case 8: lcd.print("  August "); break;
    case 9: lcd.print(" September "); break;
    case 10: lcd.print(" October "); break;
    case 11: lcd.print(" November "); break;
    case 12: lcd.print(" December "); break;
    default: lcd.print(" Error "); break;
  }

}

Sorted, Thanks