OK, so I removed the stem, locknut and cable stop and I think I discovered the cause of the problem. Apparently the front of the cable stop, which is cast with reinforcements that protrude downward fore of the section that seats against the top cup, was interfering and not allowing the stop to seat squarely on the top cup. I don't know why I didn't catch this when I initially installed it. Anyway, there isn't enough steerer height to add a headset spacer, which would have solved the problem. Instead, I filed down the interfering points on the cable stop just a millimeter or so which allows it to seat properly. I'll post a pic tomorrow.