Thanks for taking the time to answer. Unfortunately it doesn't help. With or without "Dragging" option the behaviour is the same.
I tried as well to reset the hardware and zap the PR memory. No improvements.
My next move will be to find an Apple shop and try to see the behaviour of other MacBook Air's.
In my case it was a hardware problem. I first contacted Applecare by phone (I was still within the initial 3 months after purchase). The Applecare person tried to convince me that it was a problem related to trackpad configuration and how I was using the Macbook (by the way, I am really disappoined by Applecare in general).
Then I took the MacBook Air to an Apple dealer, and they identified quickly the problem (faulty trackpad). I got the MacBook repaired the following day.
I hope this helps