Houston we have a problem

A friend of mine has a problem with ET.
Sometimes when he starts to strafe the screen looks like he has fps-lag, but it isnt because he has stable 125 all the time. (Enemies and textures are "shaking" a bit) Otherwise it works perfect but that problem makes it harder to aim.
He is using 60 disprefreshrate because it feels better, and the game runs smooth. He has tested 75,85 etc but nothing helps. He has ati radeon hd 5700. And he has tried new drivers but it doesnt help. It looks like the game is going slow.

Any help is appreciated.