Fuel Sensor

OK, I am starting to get pissy now. I just had my fuel level sensor replaced because it had a short in it where it would bounce the gage when I put it in park. Now, a couple of weeks later, it will go from just before an 1/8 of a tank to just before a 1/4 tank when I put it in park after driving for a bit. And this is on a flat surface. I used the numbers just for the sake of an argument. Please tell me if this is normal...at all. Thanks in advance.