general problems of the 2000 Buick Century

Even with occasional rotation, two tires (originally on front, and there presently), have worn extremely poorly on outside. They consistently lose air, though no leak is apparent. They now are exhibiting small cracks. The other two tires still look good and perform well, but these two now feel dangerous at highway speed, and are downright scary on wet pavement. Have there been other incidents with these tires? I feel they should hold up better than this, and don't want to become a statistic (I. E. The firestone stories). I drive carefully and take care of my car; this is the only complaint I've had with it. (some info may be incomplete; I didn't think engine size or belts were terribly important at this point. Two tires, both on front, are failing, but no further damage has been done. I'm guessing as to cost of replacement tires in the dollar amount of damages. ).