Ive gone many weekend races with the same bearings. Only time I change them is for a long a main run. Might check the hyper 8 thread see what they say, perhaps someone there has had the same issue.
It sure sounds like you checkes everything. Spin the spur around a few times, watch for any noticable drag. Perhaps the engine isnt sitting straight and thats causing this problem. If the engine isnt straight in the mounts this can cause the issue.
Hope you get it figured out, Im out of ideas for you.