I am trying to measure AC power using an Arduino with an Atmega328p. In summary, I want to use timer1 to make an interrupt every 400us where I measure two ADC channels (A0 and A1) and put them into an array. The problem is that instead of measuring five mains periods (mains frequency = 50Hz so 20ms * 5), I get instead 13 of these periods (260ms). What am I doing wrong?

volatile int sample_incremental = 0;

void setup() {
      Serial.begin(115200);
      pinMode(voltagePin, INPUT);
      pinMode(currentPin, INPUT);
      set_sample_interrupt();
      sei();
    }

void set_sample_interrupt() {
  //Count to 400us
  OCR1A = 99;
  //Mode 4, CTC
  TCCR1B |= (1 << WGM12);
  //Enable interrupt
  TIMSK1 |= (1 << OCIE1A);
  //Prescaler 64
  TCCR1B |= (1 << CS11) | (1 << CS10);
}

ISR (TIMER1_COMPA_vect) {
  // action to be done every 400 usec
  current_sample[sample_incremental] = analogRead(currentPin) - 504;
  voltage_sample[sample_incremental] = analogRead(voltagePin) - 512;
  if (sample_incremental < (SAMPLES * PERIODREADS))
    sample_incremental++;
  else {
    //Disable interrupt
    TIMSK1 &= ~(1 << OCIE1A);
    sample_incremental = 0;
    finishedReading = true;
  }
}

Where after I do one reading, I stop the interrupt, and SAMPLES * PERIODREADS = 250.