Don't think you need both of those locknuts? Maybe add a spacer and just use the one above the hanger. I also find you need to "over tighten" the headset slightly before you nip up the locknut to get the preload just right.
This seems to have done the trick. I've tightened the top race a bit more than usual, and it seems to be working, even with both locknuts still in place. Fingers crossed it doesn't just loosen itself again!
Don't think you need both of those locknuts? Maybe add a spacer and just use the one above the hanger. I also find you need to "over tighten" the headset slightly before you nip up the locknut to get the preload just right.