Pete, might try spraying some WD-40 into the bearing. It might be slipping from having oil on the rollers. You can also take a small flat screwdriver and tease the rollers around. A few may have come unseated, and if you are lucky you can tease them back into place. On the TTn, I think you may have to pull the backplate or at least the press fit to get to the bearing itself?
Lewis, one of the guys in our racing club, has the uncanny ability to fix slipping OWB by just fiddling around inside of them with a small jeweler's screwdriver in between racing. I've never been able to do it.