Time based modeling

I am having trouble implementing time based modeling in my directx game. I am using the CHiResTimer.h that comes with the opengl game programming book. Here is some sample code: ship.SetPosition(x + (800* timeElapsed)), 410); The problem is that every few seconds the speed of the ship speeds up and slows down and doesn''t stay constant. I got the code to work perfect when I was using 3d coordinates, but in 2d, it just doesn''t work well. Does anyone know where I can find some code that does time based modeling that I can use? Thanks, Chris Lynch