I think the problem is solved.
Just in case this is useful to others:
After looking at the duty-cycle reading in the X11 diagnostics connector with a portable oscilloscope, looking for a incorrect mixture setting, I found it to be a bit rich, so I corrected it.
However, I don't think that was the cause, as I also found that the mixture didn't change when the car idled with respect to low (1000rpm or so) speed, so I suspected the idle microswitch.
And voila! I found that it wasn't operated by the throotle cable lever, so when I pressed the microswitch just a bit the duty-cycle and hence the engine speed changed (I could easily hear the "click"). So I adjusted the nut of the cable and now it operates it.

I have been running for a while without problems, but I will confirm that it is solved tomorrow, when I go to work.

I'll keep you posted.