Nice feedback to my request for help, guys, i`m very thankful.
And, excluding some kid (not my dear friend killa, other... who have no education), nobody insulted me (and my mother like that kid did), because the havoc i had been causing lately with this lagging issue, thanks to all for your understanding so far.
After tried the Gian`s instructions, a different PS3, router and cables, now i know that my personal router is broken, but that has only part of the issue.
Yesterday i connected the PS3 by cable directly to the Virgin (my broadband provider) modem/router and enjoyed a race almost free from lag, only a bit here and there (sometimes i had massive lag, but only when somebody get in or out of the lobby, or when i have somebody really near me at track, and, once, a frizzed screen, some two seconds, at top of the hill and almost get fully in the gravel at hairpin, but i managed to keep two tyres at track).
Next Thursday i should receive a new modem/router from Virgin who will increase the signal from 20 to 60 mega and the problem, hopefully, should be fixed.