Trenki, YOU DID IT!!!!!!!!!!!!!
I CAN NOT BELIEVE IT!!!!!!!!!!!!

The nasty triangles completely dissapeared. I just removed the GL_POLYGON_SMOOTH, unhopefully whatched to the screen and... it happened!!. And render is now as fast as in GL_FLAT mode. This means hundreds of hours saved, and my deepest gratitude for you and the other guys who tried to help me. Ten years later... my program is now finished.

THANK YOU IS NOT ENOUGH.
But I don't know what else can I say.
Maybe in spanish (I live in Spain)

Gracias, tío, esto significa mucho para mi.
Si vienes por mi tierra, escríbeme y comerás en el mejor restaurante de Madrid.

Yeaaaahhhhhh!!!