I had the same problem. In my case the clutch fork bearing retainer was not riveted on the fork correctly. It was not letting the bearing center itself over the tranny output shaft, adding a lot of side pressure. As I worked the clutch during driving the bearing would really heat up. When I tore it down to find the problem the bearing was ready to fall apart from wear.
Not sure of the brand of fork but it was after market so there are more out there!