I replaced a pitted/brinneled headset with a new one recently. All new headset, including lower/upper races and crown race. Tange Levin alloy unit.

• Had the head tube faced by the LBS.
• Used a dedicated headset press (cheap brand but still better than any DIY method) to press in the cups.
• Adjusted the headset to have some preload once the locknut was tightened.
• Locknut is secured by at least 4 threads (i.e., steerer is not cut too short).
• Locknut is not bottomed out (i.e., steerer not cut too long).
• All preload adjustments made with the stem fully tightened in the steerer.
• Headset passed the "no clunking when rocking the bike with the front brake locked" test (i.e., not too little preload).
• Headset passed the "fork rotates smoothly" test (i.e., not too much preload, no binding at any fork position).

The headset continued to feel great for about a month, when I performed the rocking test and felt a little clunk/wobble, so I increased the preload slightly. I figured it took a month for everything to seat and this would be the only adjustment it needs for a long time.

A month later, the same dilemma. A month later, again. Month after month I have to readjust because the preload becomes too loose. I noticed that each time, the locknut is anywhere between finger loose to barely tightened.

I concluded that the locknut keeps coming loose, allowing the top race to loosen. But why? The headset-mounted brake cable stop is keyed so there is no rotation of it between the top race and the locknut. And when I tighten the locknut against the cable stop, it's pretty tight. I can't fathom why it keeps coming loose. The locknut is alloy, in case that makes a difference?

I'm considering applying some locktite to the locknut and see if that helps. Is that a bad idea?

What am I missing here?