This is a common problem with road bike headsets in particular. It is called "false brinelling" because the marks in your headset race that cause the "indexed" headset syndrome that you describe look the marks from a brinell hardness test. What is happening is, when you ride for long periods of time in a straight line, like on the road, the grease film between the ball bearing and race gets squeezed out by rocking and vibration. Once the grease film is gone, the bearing actually "welds" itself the the race, then is torn away the next time you turn. This happens repeatedly, and eventually you get pitting that lines up perfectly with the bearings in your headset. Some headsets have a bearing design which addresses this, like the Chris King. Shimano cartridge units do as well, although I'm not sure that they are still made. To revive (temporarily) an indexed headset, remove the caged ball bearings, and replace with free bearings. This way, you can add one or two more bearings to the upper and lower races, and the pits and balls will not line up. Keeping your headset clean, greased, and adjusted correctly (not too much preload) will also prolong headset life. The advice to get your headtube faced is also sound.