Bit of an update, the 990 is still fine, but I have recently traded it on a new 690. I had been wanting a 690 for about 6 months so finally bit the bullet. Regarding the 990 gearbox, the motor has to come out of the frame and be stripped/split. So a reasonably major job. The main culprit was one of the bearings that the shift drum relies on. It was binding. Various other parts were replaced as well including all 3 shift forks.
BALL BEARING 61805 C3