ein Problem beim Rauschen von ADC12 msp430f1611

Hi,
I use ADC12 of msp430f1611.but I have some problem with the ADC12
precision.
The analog input is 0(no signal), but the ADC conversion value sometimes
change of about 0 to 256 points(256/4095). But the input hat signal, the 
ADC conversion value is good and stabil.
How can I do?

Thank you
Here is the code:
void main(void)
{
  WDTCTL = WDTPW+WDTHOLD;                  // Stop watchdog timer
  P6SEL = 0X3C;                        // Enable A/D channel inputs
  ADC12CTL0 = ADC12ON+MSC+SHT0_8;    // Turn on ADC12, extend sampling 
time
                                     // to avoid overflow of results
  ADC12CTL1 = SHP+CONSEQ_3;        // Use sampling timer, repeated 
sequence
  ADC12MCTL0 = SREF_2+INCH_2;            // ref+=VeREF, channel = A0
  ADC12MCTL1 = SREF_2+INCH_3;            // ref+=VeREF, channel = A1
  ADC12MCTL2 = SREF_2+INCH_4;              // ref+=VeREF, channel = A2
  ADC12MCTL3 = SREF_2+INCH_5+EOS;     // ref+=VeREF, channel = A3, end 
seq.
  ADC12IE =0X08;                           // Enable ADC12IFG.3
  ADC12CTL0 |= ENC;                        // Enable conversions
  ADC12CTL0 |= ADC12SC;                    // Start conversion
   _BIS_SR(LPM0_bits +GIE);                // Enable interrupts
}

#pragma vector=ADC_VECTOR
__interrupt void ADC (void)
{
  static unsigned int index = 0;
  A0results[index] = ADC12MEM0;            // Move A0 results, IFG is 
cleared
  A1results[index] = ADC12MEM1;            // Move A1 results, IFG is 
cleared
  A2results[index] = ADC12MEM2;             // Move A2 results, IFG is 
cleared
  A3results[index] = ADC12MEM3;             // Move A3 results, IFG is 
cleared
  index = (index+1)%8;                  // Increment results index, 
modulo;

}