Weird graphics issue

I have a couple of games, namely Crysis and Portal, where when I move from side to side, or turn around, I get this horizontal line across the screen where it looks like the bottom portion of the screen is moving at a slightly different speed than the upper portion. In Crysis, it is more noticeable during cut scenes than during actual play, and in Portal it is not just one horizontal line, it is actually the entire screen, so if you can imagine the screen split into like 10 horizontal sections, section one moves first, then section 2 and so forth and so on, each section moving at a slightly different speed than the others.

I hope that someone out there can understand me, and if not I can try to explain it better.

So, I have an Ati Radeon HD 5850 graphics card, and in both Crysis and Portal, the video settings were both set by the game itself, like auto detect. Also, a game like Mass Effect 2 and Dragon Age both look perfect, even with the settings set to high. Also this problem is not a major problem, it is not stopping me from playing these games, but I would like to make it better if I can, so I am wondering if there are some settings that I can change, or what settings can I change to help fix this issue.

tearing is occuring, make sure your monitor display is set to max refresh rate (normally 75Hz) and in the game options the same is set, also enable VSYNC as recommended by wishmaster....if it still occurs update your game and also your video drivers and try to play the game at medium settings and then increase the graphics manually.!!

I wanted to thank both of you guys, I checked on my refresh rate and it is set to 75Hz, and I went in and turned on the vsync, and it fixed the problem in both Crysis and Portal, and I didn't have to lower the resolution in either game. So thank you both for your help.