Thanks a lot for your suggestions, but I still haven't solved the problem. I disabled from the BIOS the PnP feature, but I haven't found the "assign IRQ to AGP" voice on my Award BIOS.
The problem is always the same, I post you also the kern.log file now (for the unchanged XF86Config-4 and XFree86.0.log file refer to the previous post).
My IRQ configuration, if useful, is
IRQ 3 : Serial 1
IRQ 4 : Serial 2
IRQ 5 : Parallel port
IRQ 7 : PCI slot 1&5 [PCI5: SBLive! 5.1 - PCI1: empty]
IRQ 9 : PCI slot 2 [empty]
IRQ 10 : PCI slot 3 [internal 56k modem Conexant]
IRQ 11 : PCI slot 4 [Realtek ethernet card]
If you have any tips on what to try or what to do to find or solve the problem, you're obviously welcome...
Thanks for your patience, I'm going crazy!