We had a similar problem in 2012 with sharp IR sensors from Pololu electronics.
http://www.pololu.com/catalog/product/1134
They are inexpensive and worked very well at first to control the indexing of the balls in our pick up. Between the 1st and 2nd competition we change the robot covering for a couple of reasons. That's when we started getting false positives. At that point for many reasons we could not go back to a robot covering that shielded the sensors. The human operator ended up being the controller. A tough lesson. So in the future we would use them again but design shielding in to the robot.