Rear hub cones keep loosening
First the history:
Built a new set of wheels for my commuter. I used a NOS Shimano 600EX hub I purchased on eBay. It was new in a box and had never been laced. Took the cones and bearings out just to see the condition of the grease and it was fresh (not dried out). I added a bit of extra Phil Wood grease as it went back together, adjusted everthing correctly, built the wheel, and began to ride it.
Now the problem. Every hundred miles or so I notice that the cones seem to be loose. There is excessive play in the bearings. The first time or two that this happened I just snugged the adjustment up and rode on. It has happened three times now so I dismantled it and had a look. The bearings are all there and still look like brand new. The cups and cones are like new too and no visible signs of wear. The axle is straight and is not cracked/broken. I washed out all the grease this time and regreased with new Phil Wood grease. Again checked the bearing play as it went together and it is perfect. As before the bearings feel very smooth. I've checked each time to ensure I didn't forget to tighten either of the cone locknuts.
I'm at a loss on this one...any suggestions?