Garth, the only tip I can come up for you, since you say you are being careful, is that you may be adj the cones too tight. I find that even if I rebuild with a rough (relative to new) race and/or using old balls, if I don't apply any preload at all then the bearing feels fine in the hand. However the converse, if adj pristine bearing and have the slightest preload, then the bearing is rough in the hand. Don't forget that tightening outer (mounting) axle nuts or quick-release tightens the cones as well. Some may disagree with my method, but zero clearance (the perfect adj) only applies in theory. In practice, its preferable to be loose rather than tight. Ball and cone bearings allow for a great (relatively) latitude of misalignment, but only if they are loose enough to be able to move in response to misalignment.
Jack Romans
Sacramento, CA