Probleme mit Overflow ATtiny44-20

/* PA0 Taster

 * Channel 0 PA1 Timer0 Start 

 * Channel 1 PA7 OC0B PWM

 * Channel 2 PA2 TCNT0 Wert

 * Channel 3 PA3 ISR Compare

 * Channel 4 PA4 ISR OVF

 */

//-------------------------------------------------------------------------

#define F_CPU 1000000UL    // ATtiny44-20 with 1Mhz

#define Timer0_stop    TCCR0B &= ~((1<<CS00)|(1<<CS01)|(1<<CS02))

#define Timer0_start    TCCR0B |= (1<<CS01)

#include <avr/io.h>

#include <avr/interrupt.h>

#include <util/delay.h>

//-------------------------------------------------------------------------

ISR(TIM0_OVF_vect)

{

  PORTA |= (1<<PA4);

  _delay_us(100);

  PORTA &= ~(1<<PA4);

}

//-------------------------------------------------------------------------

ISR(TIM0_COMPB_vect)

{

  PORTA |= (1<<PA3);

}

//-------------------------------------------------------------------------

int main(void)

{

  cli();            // clear Interrupt

  DDRA &= ~(1<<PA0);        // PA0 Input

  PORTA |= (1<<PA0);        // Pullup on

  DDRA |= (1<<PA1)|(1<<PA2)|(1<<PA3)|(1<<PA4);  // PA1 - PA4 Output 

  DDRA |= (1<<PA7);        // PA7 OC0B

  TCCR0A = (1<<COM0B1)|(1<<WGM00)|(1<<WGM01);  // fast PWM

  TCCR0B = (1<<WGM02);        // fast PWM

  OCR0A = 250;          // Top                  

  OCR0B = 125;          // Compare

  TIMSK0 = (1<<OCIE0B)|(1<<TOIE0);    // Compare und OVF

  sei();            // set Interrupt

//---------------------------------------------------------------------------

  while(1)

  {

    if(!(PINA & (1<<PA0)))

    {

      _delay_ms(1000);

      TCNT0 = 0;

      PORTA |= (1<<PA1);

      Timer0_start;

    }

    if(TCNT0 == 125)

    {

      PORTA |= (1<<PA2);

    }

  }