RTC Problem in LPC1768

Hello, I'm having problems with RTC in LPC1768. The RTC is running, but is advancing approx 5 seconds per hour. The electrical circuit is good, with a 20ppm crystal tolerance, and 12,5pf capacitors of load. Tha VBAT is conected in CR1220 lithium battery. I have reviewed the source code and is OK. Below is my code:

void InitRTC(void) {
  LPC_RTC->CCR  =    ( !CLKEN |  CTCRST );
  LPC_RTC->CCR &= ~CTCRST;
  return;
}
/***************************/
void StartRTC(void){
  LPC_RTC->CCR |=    ( CLKEN );
  return;
}
/***************************/
int main (void){
  char temp[36];
  InitRTC();
  StartRTC();
  while(1){
    sprintf ((char *)temp,"%2.2d/%2.2d/%.2d   %2.2d:%2.2d",LPC_RTC->DOM,LPC_RTC->MONTH,LPC_RTC->YEAR,LPC_RTC->HOUR,LPC_RTC->MIN);
    lcd_gotoxy(1,0);
    lcd_print(temp);
  }
  return 0;
}

The RTC is running correctly without the battery, with main power connected in VBAT. But yet the RTC is running advancing. I can't use the calibration of application note AN10849. I have 100 identical hardwares, and they are with different accurate. I think that this variation is very high for be simply a low accuracy of LPC. There must be some other problem.

Does anyone know what might be happening?
Thanks