Is something wrong with my graphics card?

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.

Specs:
CPU: E8400@3ghz
4gb ram
Vista 64 bit

turn vsync on in your games for one.
are you sure all of your ram is being recognized?
defrag your harddrive.

those suggestions are pretty lame but won't hurt the situation.
it sounds memory related, have you overclocked?