This Capacitor makes me MAD !!!

Ich verstehe nicht - warum ist so?
I have soldered a very simple oscillograph based on Atmega8. ADC in the 
microcontroller measures voltage between +5 and the ground. This info is 
being sent periodically through the serial line to PC.

All is ok. For example, if I use simple voltage divider (R1 and R2 in 
series) and measure voltage between them, it shows me really nice exact 
values.

But, as soon as I have capacitor in a circuit, all goes wrong. I know 
how capacitors work and how  they should work. But in my case, a 
capacitor makes measured values to swing around the real voltage. Up and 
down, up and down...

What the hell is it ? Any good oscillograph doesn't show something 
similar ever. I suppose it is ADC issue inside Microcontroller.

My question is following:
How it is recommend to use ADC in the Atmega in order to measure any 
outer voltage?

My ADC function looks so:
void ReadChannelInBuf(uint8_t mux, uint16_t *buf, int len)
{
  uint8_t i;
  //Frequenzvorteilersetzen auf 8 (1) und ADC aktivieren (1)
  ADCSRA = (1<<ADEN) | (1<<ADPS1) | (1<<ADPS0);

  // Kanal waehlen
  ADMUX = mux;

  // interne Referenzspannung nutzen
  ADMUX |= (1<<REFS1) | (1<<REFS0);

  for(i=0;i<len;i++){
    ADCSRA |= (1<<ADSC);
    while ( ADCSRA & (1<<ADSC) );
    buf[i] = ADCW;
  }
  // ADC deaktivieren (2)
  ADCSRA &= ~(1<<ADEN);
}


And I do measurements in the loop:
int main(void){

    usart_init();
    uint8_t i;
    uint16_t result;
    uint16_t *results;

    results = (uint16_t *)malloc(sizeof(uint16_t)*LEN_BUF);
    while(1) {
        ReadChannelInBuf(0, results, LEN_BUF);
        for (i=0;i<LEN_BUF;i++){
          result = results[i];
          usart_putchar((uint8_t)result);
          usart_putchar((uint8_t)(result>>8));
        }
    }
    return 1;
}

Please help me,
Thank you in advance and I am very sorry for my German, but my English 
is a way faster for me.