I run a PIII on a PowerMac G4 using an Adaptec 2930.
I also have the
Ratoc FR1SX scsi>firewire adapter (with optional AC adapter and High-Density 50F to Centronics 50M Adapter) which while it works, for some reason I've never been able to determine, it disables the other firewire ports, which means that my external HDs are not accessible if the scanner is on.
Supposedly the only difference between the PII and the PIII is that the latter came with the scsi>firewire adapter.
I have also used this scanner connected to a laptop PC using an Adaptec SlimSCSI 1480 cardbus adapter.
Assuming there is not some other underlying problem with the computer itself, your choices would be either to replace the scsi card or go with a scsi>firewire adapter.
You might also want to check out the Imacon forum on Yahoo:
http://groups.yahoo.com/group/imaconusers/
Good luck with it,
Keith