i tried all sort of different settings, as well as playing with the function editor. 

i always get a very pixelated reflection in the floor, even at final rendering settings.
check the cropped image. notice the difference between the plane and the floor. 

do you know how i can solve this?

thanks a lot,
alberto

What are your render settings?  If you are just using the Final preset, that's not enough.  You need to bump up the AA settings.

I use a min/max subray 6/12 and set the quality threshold to above 85%.  Then do a small test render area and see if it looks better.  Slowly bump up these settings and see if it helps.

Looks like you have blurred reflections, and if so, you need high AA settings to get rid of the noise the blur produces. As Peggy said, Final is far from enough.

i have been trying on small parts. 
settings at 6/12 and 85% not so good.
then i tried 60/120 and 85%. still not there (see attached).

brought it up to 90% and it is taking forever. i might have clicked something else...:)

anyhow, trial and error i am moving forward (slowly). any other tips very welcome.
thanks a bunch.

Object, no need for texture AA here. If you don't need blurred reflections, just put back the parameter to zero in the advanced material editor.

Not sure, but it if u use a bump map that could influence it too, as well as specular.   Try resetting those to zero and see if it makes a difference.