Any ideas on why it's jittery or any ideas on what to do about it?
Jitter is to be expected on a sensor like this. Just how much jitter are you seeing?
You can try to filter out the jitter, either electrically (RC filter is the simplest) or in software, using something simple like averaging N samples or a bit more highbrow like an IIR filter (there's an example IIR filter C++ class implementation in our Gadget Shield library code -- look at the Accelerometer demo).
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Beat707: MIDI drum machine / sequencer / groove-box for Arduino