Smooth animation

Dear all,

I have a problem my my 3D app, I have tried to simplify the problem, and
felt into working back on the tutorials. Here is the problem :

I am working with the DirectX SDSK tutorial No6.
Instead of making the tiger rotate, I make it translate horizontrally on the

The problem : this animation is not smooth.
In spite of it is fast (more than 100 Fps), it looks like the tigers stops
very shorty twice or more per second. It is not easy to see it, but it
really gives the feeling that the app is not "smooth".

- I have try to display the app fullscreen, same problem.
- I have tried to set 2 to the Backbuffercount (Tom Millers says in his
"Managed DirectX 9 Graphics &game programming", that wotking with backbuffer
ocunt > 1 will improve "fluidity"), without result.
- In fact, I have tried all the parameters without successs
- I have also tried to turn off almost all the processes working on my
machine... No result.
- I have tried on another machine, with another graphics card (mine is
NVidia 6x, the other is an ATI). Same result!

Does anyone has encoutered the same problem?
Is there a trick to avoid it?
In advance, thank you!