Strange results HC-SR04

Hey, I recently purchased a HC-SR04 ultrasonic sensor off eBay (cheap as chips!) and after trying both the NewPing library AND pulsein method, i'm getting very strange results! If I put my hand or an object stationary in front of the sensor, it reports very erratic values, normally oscillating between 0 and what would seem like a reasonable value of distance. Is my sensor faults and should I send it back or is it a fault elsewhere?

Thanks,

can you rewrite your code to a minimal sketch that shows this behaviour and post it?