I have an HD4850, and it seems that in some of my games the game will freeze for a split second whenever the fps goes down.
For example in Half Life 2, say there is a small room in front of me, and a big room behind me. And lets say that when I am facing the small room I get an fps of 150 and when I am facing the big room I get an fps of 80. What happens is I will be facing the small room and getting 150 fps, but if I turn around towards the big room causing my fps to drop down the 80, the game will almost always freeze for a split second.
This happens also in Company of Heroes. The game runs smoothly at 120 fps, but if I zoom the camera in and then look straight out across the world, causing the game to drop to 30 fps, it will almost always freeze.
I also have this problem in Halo 2.
Yet, despite the problem, I don't notice it at all in Team Fortress 2 or Crysis.
I don't understand what is wrong. Does it sound like something is wrong with my video card or what? It is not too late to exchange it.
I think such intermittent freezes happen all the time, no matter how powerful your system is, having lots of ram (minimum 2GB) is always good, but make sure your drivers are up to date. But like u said even if it does fall from 150 to 80FPS you shouldn't be able to notice any stuttering. The above mentioned solutions should also be given a look at.