I was totally fine playing HL1, and HL2, and HL2 episode 1.... but I never finished episode 2 because of motion sickness. The problem isn't really with episode 2 though. The problem is just that I got old, and now I get motion sickness from FPS games that didn't affect me before.
But I do know that not every FPS makes me sick. I think mouse-look smoothing helps. I'm not certain what else, but I'd try messing with the field-of-view angle and stuff like that.
Have you tried enabling and opening the developer console and changed the sv_friction value to something higher than the default of 4? Perhaps 6? Might get rid of the sliding of the character movement and help with your motion sickness.