The problem seems to be solved.
I tried to desactivate the VSync in the In-Game menu but I couldn't see any improvement so I started to play a little bit with the other settings until I finally set "Anisotropic Level" from "3" to "Off" and "Textures" from "Auto" to "16". Now the game runs very fluid with every level of detail and crowd, from empty stadium to fully crowded. I tried to reset my Graphic Card options to default and the game still runs OK so, basically, In-game VSync, Anisotropic Level and Textures were the only values to modify, it was not a Graphic Card problem. Thank you