Erratic shooting with Gamo Whisper?

I've had my Whisper for about 8 months now and I've mostly shot it with just iron sights. Lately I've put my scope on it and I sighted it in. However for some reason it doesn't keep it's zero. I have all the screws nice and neatly tightened down, and the scope doesn't wobble or anything. I get really erratic shots completely randomly. I may have a good group of 5 going and then on the 4th shot it goes 2 inches low and 1 to the right, and the 5th is back on target, what could cause this?