I have identified the problem and fixed it!
 

The twisting of the steering on my Mu P8 Dahon is real, it's dangerous, and it's not a failure to adequately tighten the clamp. The problem is the quick release clamp, which uses an aluminum lever. I was discussing the twisting issue with a mechanic friend, who reminded me that when aluminum levers first started being used on wheel skewers about 20 years ago, we quickly found that they couldn't hold a wheel in a horizontal drop-out. They were only strong enough for a vertical drop out where the dropout was used to resist the twisting motion on the axle. To fix the problem with my steering, I simply found an old steel seat-post quick release and an appropriate collar, and ..... INSTANT FIX. The bike went from almost unridable to rock solid. Please note: I also replaced the collar, because the old steel quick releases were intended to mate with a flat surface, and the Dahon collar meets the quick release with a curved surface. You might be able to salvage the Dahon collar by filing down the curved surface, but I had access to a box of spare collars, so I didn't need to go that route.
I've been trying to figure this out for two years, and when I did, I specifically joined Bike Forums so that I could get the word out on this issue. Please pass it along.

I have an older Curve that had a handlebar post recall from CPSC.
Was this rotation the reason? I never got an answer.
I have not had rotation problems with the post, and have found the Dahons pretty solid and not overly flexy.
The do take a watchful eye to check adjustments, but one should be doing that on any bike.
Get in a hurry, get in a problem.


God Bless Thor