"Elisa Francesca Roselli" wrote: (clip) I have taken to cycling the 9 km to
my workplace on my newest bike, Flyzipper, a Dahon Impulse P21. (clip)
^^^^^^^^^^^^^^
It may be inherent in the design of the bike. I am not familiar with that
particular Dahon model, but I had a scary experience on my Dahon folder
while going fast downhill. That particular bike had a fairly vertical fork,
resulting in very rapid steering. At high speed it was difficult to
maintain course without overcorrecting. And, of course, each overcorrection
results in a corresponding even larger counter correction, which is the
essence of instability.
If that is what's going on, you may have to limit your speed, and tighten up
your "internal feedback."