Thank you all for the suggestion
I had a very drastic approach: I built a spare firewall with new mobo, new HD and new NICs.
I loaded pfsense 1.2.3 and my configuration.
In office non-working hours I plugged in and tested / stressed for few hours with no problem at all from various workstations. I even increased the download speed to 15/20 MB/sec (depending from the OS of the workstation).
Yesterday I definitively changed the firewall with the new one.
The old firewall (Pentium III 800EB + shuttle mobo) has been on 24/24 from 2001, working as a PC, as a little server, as a Winroute server, as a monowall firewall and lately as a pfsense server.
It still seems operational, but probably the NICs have suffered most.
Max