adc help</b)

hey friends
i am using adc 0.1 . it is connected to a potentiometer . my problem is that if i read say 1.75 volts in using the adc than it takes at least 2 reads to get in the correct reading . for eg say the voltage is 1.25 and i change it to 1.5 then the first reading after the chage will b say 1.35 then 1.44 and then 1.5... can any one tell whats the problem ....

the following is my code

IOSET0=IOSET0|0X00010000;   // i use a push button to get in the reading
while(IOPIN0&0X00010000);

AD0CR = 0x01210a02;
while(( AD0DR1  &0X80000000)==0);
        adcdata3 = ( AD0DR1  &0X0000FFC0);
                adcdata3 = adcdata3 >> 6;

thanks