With 2.6.8 you have:
> ACPI disabled because your bios is from 95 and too old
> You can enable it with acpi=force
> IRQ9: eth0
With 2.6.18, on the other hand:
> ACPI: Core revision 20060707
> ACPI: setting ELCR to 0800 (from 0200)
> ACPI: bus type pci registered
> ACPI: Interpreter enabled
> ACPI: Using PIC for interrupt routing
> pnp: PnP ACPI init
> pnp: PnP ACPI: found 12 devices
> PnPBIOS: Disabled by ACPI PNP
> PCI: Using ACPI for IRQ routing
> PCI: If a device doesn't work, try "pci=routeirq". If it helps, post a report
> IRQ11: acpi, eth0
I would therefore try booting the 2.6.18 kernel with "acpi=off" and/or
"pci=routeirq" appended to the kernel parameters.