emacs scrolls too fast when mouse-dragging selection

[nospam55]

Hi 

from many months I suffer a misbehavior of my new emacs on redhat 8. 

I click and drag down with the mouse on the window to extend selection:
when I reach the top margin of the window then the text in the window scrolls 
upwards in order
to make visible new text down ; all of this is normal behavior, of course; the
problem is that right after the first lines , the scrolling becomes suddenly too
fast to control, and I "crash" quickly on the end of file, almost always far too
down for the amount of text I wanted to select.

How can I get a smoother scrolling ? 

 thank you for reading :-)