The blade is continuing to spin, though the arbor is slowing down, thus loosening nut. When teaching, we had a Delte 14"RAS with electronic brake. It did the same thing. Check both blade and arbor flange for waxes or oils. Replace arbor nut and flange. I bet one of the Delta combination nuts and flange washer will fit Griz. You knew when you bought this saw, it was going to be a BEAR to fix. If memory serves me correct, this is a cabinet saw, similar to Uni. If so, remember to take a measurement with dial indicator from miter slot to side of blade. Reinstall with same measurement, or your miter slot based jigs no longer will line up. Or you could just build all new jigs.