Google's just testing you out, my friend.
Trust me, it's happened to all my sites. One week you rank #1 for one phrase then you drop off for a month or two. But overtime as you continue to build traffic and acquire backlinks Google will begin to see you are here to stay and your rankings will stabilize.
The customers in the SBI! forums are always claiming the same thing but that always happens with newer sites in the beginning. My flat stomach site used to have the same issues. Just hang in there!
