Realtek NIC's are junk, but with that said, many times this can be fixed with a BIOS upgrade or changing BIOS settings (like setting PNP OS to no).
Realtek NICs are junk, that's all. I tried all BIOS settings, changed slots, changed IRQ: nothing!
Thanks to Manuel Kasper's advices I bought 2 Intel NICs, 2 pro 1000 and all's going great now.
I payed 60 euros for them but they really worth that sum!
Thanks, bye.
Michele.