Thank you for all your replies, I'll take time to check everything and make consistent replies. Thor is completely right, this is an used rental bike, so despite being carefully maintained, it has for sure seen some abuse. I didn't thought about that rear hub...
Yeah there are a lot of things I don't like about the bike and a lot can be tweaked, or I could live with even if I find it quite annoying. But the really most important is that mysterious rear train instability... I agree that I should try another tire. These tires I had never heard of are incredibly cushioned, it's a "shock" (absorber) compared to the Dahons I test rode from shops. That said, I never tried Big Apples, I only rode Marathon... And I've been pleased that with these I could handle potholes more or less the same than with full sized bikes. The point with cobbles is not comfort, which is ok. It's keeping decent control...
Will talk to the biggest Dahon shop of Paris which is only a few kms away. He's known to be a very competent mechanics. But not today... (not on a Saturday!).
Last edited by NeoY2k; 01-07-12 at 05:05 AM.