Perhaps the QR sliding surfaces are worn? With lower cost QRs, the plastic may wear through and them you have lost the resilient block that is an important part. The movements from riding the bike and pulling on the bars can easily work it loose if there is no 'give'.
Also, maybe some mild Loctite in the thread to prevent it from turning?
1. The QR's sliding surfaces do appear to be worn, which is puzzling; how could they be worn? I'm thinking I may be able to fix the problem with a flat file; if the parts don't mate perfectly there will be movement that may be causing the problem. But removing metal seems a bit drastic....
2. Yes, good suggestion, I'll have a look at that resiliant block; I'm pretty sure it is not missing, but it may have lost some of its resiliance over the last 33 months and 7000 miles.
3. No, Loctite is not the answer; the QR adjusting nut is not turning.
Above all, though: I'm wondering if anyone else has noticed this!