Firewire diagnosis utility?

Does anyone know of a utility application that can diagnose a firewire chain of devices? I have a chain of three Firewire 800 hard drives that have their own power supply. While I'm not getting any errors in read/write to the drives I am wondering if the iMac's power supply is just not up to snuff.

So what is the problem you might ask. I'm trying to eliminate the use of external Firewire drives regarding iTunes audio stuttering/skipping. This has been suggested as a possible issue by AppleCare support.

I also have a USB drive plugged in and USB speakers. I don't always have those running, but I suppose this further taxes the iMac's power supply.