fog problems on ATI Mobility 9100 128MB

fog works at less than 3 miles vis. but when I bump the visibility up more than 3 (say between 3 and infinite) It shows no haze or fog at all.. it is perfectly clear, but the horizon is drawn at approx distance that Ive set. anyone heard of this before?
My CPU SPEC a P4 3.0ghz 1 gb RAM 60GB HD

That's just the way it works. If you set the visibility past three miles then there is no fog, it won't do haze or slight fog.

I may have explained myself wrong because it wasnt doing this before.. there is no fog or haze effect on the horizon like there should be. I see a line with sky above as if the computer isnt drawing the graphics at all.. It doesnt do this in safe mode, in safe mode I see a more realistic horizon with a haze effect withmuch slower frame rates, in normal full screen mode it shows no haze graphic at all, I believe it has something to do with direct 3D or I have a serious problem with my 3D card also frame rates are much lower than they used to be even in normal mode.