Tom, congratulations on the new iMac G5! If, I recall, you either never installed the third party firmware update, which fixed the problem for me, or you did install the update, but did not report it. For Marty's benefit, there is a third party firmware update for this drive, that goes beyond the official update. Pioneer is not real good at updating Mac drives, so a third party has taken this step for it.
Below are a couple of links to supply you with more info on the topic. The first site is a French site, in french. You can use Google's transalation tool, which transalates websites if you do not speak french. It helps a little. The second link is one that explains a little bit about the drive. These sources will provide you with a good start to see if you need to update your drive.
The Apple firmware installer will likley tell you you do not need to update the firmware because you have the latest "official" version of the firmware. This does not mean the firmware is not your problem, it just means the computer sees that you have the latest offical version of the firmware installed, regardless of whether it is defective or not.
If it turns out you cannot use this third party firmware update (as some people report is the case who have internal Apple supplied drives), your only option probably is trying to find the lowest availalbe write speed disks you can find.
Good luck.