Assuming the headset is fitted and not moving and its clamped up by the stem then its working loose because:
a) lack of pre-tension
b) something moves/worn bearings from above
If the frame is SLIGHLY ovalised then a bit of bearing retainer can help.
Pre-tension .. have you got shims above the bearings? and if not do you need them? Is the top cap able to give proper pre-load into the bearings?
Is there enough gap to take up slack/preload... if you set and remove the top cap is there still 1-2mm clearance?
Is everything properly settled... proper rock forwards and backwards weight on wheel
Are all your spacers flat... if they are slightly thinner or thicker and rotate that would loosen
I'd remove the problem solvers thing... its just complicating matters
Something moves ?
Is it internal .. are the bearings worn so pack down as its in use?
Your challenge here is because you changed stuff and it kept coming loose your chance of ovalizing or wearing bearings etc. is increased so you might be to some extent chasing moving goalposts?
