PingouinMalin's post in Totally solved (thanks to the heroes of STEP) - STEP Extended (full) : horizontal lines in the distance (far away) was marked as the answer
Well I tried to uninstall and, no surprise, it did absolutely nothing. I tweaked hte profiles, and nada.
I was unconvinced by the Graphic Card drivers but still uninstalled it (actually uninstalled the whole card by mistake) and...
VICTORY IS MINE !
It was either the drivers or NVidia Inspector, which I deleted too.
Big, big thanks to all the people who took time to help me. Really nice of you all.
Now I can ruin my Skyrim again with mods (hopefully not) !
