Need help with a data logger

Hi: I have been working trying to built a data logger, I want to record the date and time when a device was turn on and off. I currently have the Arduino Uno with the data logger card (SD 4Gb card) configured and it stores data, I have connected a 12 volt device through a voltage regulator tha supplies an output of 5 volts and it starts recording the information when this is connected to a digital input on the Arduino (digital input 7). After the device is off it stops recording and when it is on it starts recording again. The problem that I have is that records for certain time and then it stops even though the device is on again, also in some cases it stores garbage.

I dont know what causes this, but any help is very well apreciated.

Regards

CarlosH.

Here is the code that I am using:

/* Programa para detectar un voltaje y Grabar con Tiempo y Hora la Actividad hasta que se el voltaje sea cero
*/
//Rutina para mandar llamar las librerias
#include "Wire.h"
#include <SD.h>
#define DS1307_ADDRESS 0x68

// Definicion de la variable del puerto Analogo para lectura del disparador
int Arranque = 7;
int chipSelect = 8;
int CS_pin = 10;

void setup ()
{
  // habilitar el puerto serial para ver pantalla
Serial.begin(9600);
          Serial.print("Initializing SD card...");
  // Poner el pin de la tarjeta como salida
pinMode(CS_pin, OUTPUT);
       
        Wire.begin();

// poner el pin 0 como entrada Analoga
pinMode(Arranque, INPUT);

if (!SD.begin(chipSelect))
{
  Serial.println("");     
  Serial.println("Card Failed");
       
        return;
}
        Serial.println("");
        Serial.println("Card Ready");

}

void loop()
{       
    

  if (digitalRead(Arranque) == HIGH)
  {
    printDate();
  }
 
  else
 
  if (digitalRead(Arranque) == LOW)
{
  Serial.println ("Motor Apagado ");
        }

}

byte bcdToDec(byte val) 
{
// Convert binary coded decimal to normal decimal numbers
return ( (val/16*10) + (val%16) );
}

void printDate()
{
// Reset the register pointer
  Wire.beginTransmission(DS1307_ADDRESS);

  byte zero = 0x00;
  Wire.write(zero);
  Wire.endTransmission();

  Wire.requestFrom(DS1307_ADDRESS, 7);

  int second = bcdToDec(Wire.read());
  int minute = bcdToDec(Wire.read());
  int hour = bcdToDec(Wire.read() & 0b111111); //24 hour time
  int weekDay = bcdToDec(Wire.read()); //0-6 -> sunday - Saturday
  int monthDay = bcdToDec(Wire.read());
  int month = bcdToDec(Wire.read());
  int year = bcdToDec(Wire.read());

  //Formato de la fecha y hora  3/1/11 23:59:59
  Serial.print(month);
  Serial.print("/");
  Serial.print(monthDay);
  Serial.print("/");
  Serial.print(year);
  Serial.print(" ");
  Serial.print(hour);
  Serial.print(":");
  Serial.print(minute);
  Serial.print(":");
  Serial.println(second);

// String para grabar info a archivo
   String dataString = String(month) + "/" + String (monthDay) + "/" + String (year) + " " + String (hour) + ":" + String (minute) + ":" + String (second);
// Apertura y Nombre del archivo
   File dataFile = SD.open("datalog.txt", FILE_WRITE);
  
   if (dataFile)
    {dataFile.println(dataString);
    dataFile.close();}
   
   }