Durability of HC-12 Modulus

Hello,
I use Arduino pro mini 3.3V, HC12 modulus to transmit my data and have a problem.
At the beginning the modulu in baut rate 9600 works fine, I received data. After only two hours it did not work. All the data from HC12 are garbled. Then I turn to command mode, send "AT" and the response is also garbled.
I had tried to adjust baut rate. I had lowed baut rate from 9600 to 1200(or2400)). It worked but after 1 hour it happened again. All responses, including command mode were garbled. I have 4 chips and each chip has the same problem.
I will be appreciate for any advise if some one has experience of this HC112 Modulus. I do not find such problem in the internet.
Thanks

Please post your programs (for the sender and the receiver) and also post diagrams to show clearly how you have everything connected. A photo of a simple pencil drawing will be best. See this Simple Image Posting Guide

...R

every one I tried failed in minutes. send it

//based on code from: Tom Heylen

#include <SoftwareSerial.h>

#define rxPin 2
#define txPin 3
#define stPin 7

SoftwareSerial HC12(rxPin, txPin);
long baud = 9600;

void setup()
{
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(rxPin,  INPUT);
  pinMode(txPin, OUTPUT);
  pinMode(stPin, OUTPUT);
  Serial.begin(baud);
  Serial.println(); Serial.println();
  digitalWrite(stPin, HIGH);
  Serial.println("HC-12 Test & configure");
  HC12.begin(9600);
  Serial.println("HC-12 started"); 
  if (HC12.isListening())
  {
   Serial.println("HC-12 is Listening ");
  }
  else
  {
    Serial.println("NOK");
  }
  {
   if (!HC12.available())
   Serial.println("HC-12 is not responding");
   //while(1);
  }

  //test HC-12
  digitalWrite(stPin, LOW);
  delay(1200);
  Serial.println("Sending AT+RX");
  HC12.print("AT+RX");                
  Serial.println("command sent");
  Serial.println("setup end"); Serial.println();
  delay(500);
  digitalWrite(stPin, HIGH);

  if (HC12.available())
 
   {
    Serial.print("HC12.available()");
   }
   
  while (HC12.available())         // If HC-12 has data
  {
    Serial.write(HC12.read());     // Send the data to Serial monitor
  }
   digitalWrite(stPin, LOW);
  while (Serial.available())       // If Serial monitor has data
  {
   HC12.write(Serial.read());      // Send that data to HC-12
  }
   digitalWrite(stPin, LOW);
}                                                                                                       
void loop()
{   
}

it reads once and never responds again. I tried using a 3.3V level converter on the TX side. no effect