Unstable Frame Rates

programming flash
1

Why is Flash’s frame rate so unstable? I have set it to 60 FPS and here is my output frame rate:

FPS: 76.92307692307692
FPS: 500
FPS: 58.8235294117647
FPS: 83.33333333333333
FPS: 55.55555555555556
FPS: 55.55555555555556
FPS: 71.42857142857143
FPS: 55.55555555555556
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556
FPS: 62.5
FPS: 62.5
FPS: 55.55555555555556

The frame rate is measured by code in my other [COLOR=“Red”][SIZE=“3”]thread[/SIZE][/COLOR]

And I am not doing much, only a tiny physics engine simulating 1 object moving upward with no other forces other than initial acceleration which is (0, -100)