Sonar/PING only reporting 14cm or less

I'm having an intermittent problem with some PING sensors. I'm just using the example code and have 3 copies of the circuit and exhibiting strange behaviour. From time to time, they only report values up to a max of 14cm. I've also confirmed that a colleague has had this happen occasionally too with his experiments with the PING.

This is very strange. Has anyone ideas of why this might be happening? The circuit is fine, but could there be interference, how could I track it down.

Any ideas?