I have a threaded 1" headset and fork. The steering has developed a "notch" at dead-straight ahead - it takes a little bit of force to get it off-center, but then it is completely free side-to-side. At first I figured it was the typical "indexed" steering that occurs when the balls wear little low spots into the races. But I'm not so sure for these reasons:
1) This is an old headset with a certain amount of miles, but nowhere near it's typical limit - this is a Campy Nuovo Record unit with a reputation for everlasting goodness. I just installed it on a new bike about a year and a half ago. It was fine then and this problem came on quickly and has gotten worse quicker than it seems like it should.
2) There is no next-index over. There is only the one "stop" at dead center.
Could something other than wear in my headset be contributing? Could it be an issue with alignment of the cups and races?