Infrared Sensor Reading Problem

http://store.extremeelectronics.co.in/Line-Sensor-Array.html
@AnalysIR
This kind of sensors. I am having problem regarding the threshold. Suppose the robot will go forward when the sensor read 0 1 1 1 0. here 0 means white surface and 1 means black. But when i ran it on the field the ir value jumps a lot and so the robot can maintain its orientation :/. Any suggestions ?