Problem using ultrasonic sesor

Hey guys,
I was using ultrasonic sensor to measure the distance, yesterday it worked well and measured distance upto 3-meters. But today I used the same program to measure the distance today, it measured distance up to 65cms, but more than that it is showing 68cms no matter how far I put the object from the sensor,(i mean after 65-cms). I tried using 6 different sensors and the arduino board, still the same problem even I restarted my computer, still the same problem. But when I tried one of my friend mac it worked properly for the same code. but not on another friend's windows. Ia am using the windows btw.
Can anyone please explain why is this happening.
Below is the code:

#define trigPin 13 
#define echoPin 12 
void setup() {
  // put your setup code here, to run once:
Serial.begin(9600);
pinMode (trigPin,OUTPUT);
pinMode(echoPin,INPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
float duration,distance;
digitalWrite(trigPin,LOW);
delayMicroseconds(2);
digitalWrite(trigPin,HIGH);
delayMicroseconds(10);
digitalWrite(trigPin,LOW);
duration=pulseIn(echoPin,HIGH);
distance=(duration/2)*.03435;
Serial.print(distance);
Serial.println("");
delay(100);
}

Some of theeze sensors are sensitive to Vcc. There are at least 3 types of, with different xtal and comparator

The one with a 4MHz xtal is ok @4.95V, but not @4.8 or 5.0