I need to detect shaking (not just vibration or small movements but significant horizontal rocking movement)! (its a long story) 

I have an Arduino already in place, controlling some other stuff. I've tried a couple of simple methods with gyroscopes and pendulum like shake sensors and like, but they are either too sensitive or to hard to calibrate. 

Mostly I need to detect horizontal plane movements. 

Any suggestions? Anyone had done anything like this?