It could be that the jump happens when the drives have there caches flushed by the system. I know with the Xserve RAID there is an option to disable this so the cache is manged by the hardware controller and not the Operating system. The difference is quite amazing. I don't know if this is an option with your SCSI drive (I don't use one) but I have noticed this same effect when playing back longer files off Raptors.

Unfortunately I think Quicktime controls all this stuff so it is out of our mere mortals control.

This is just a guess so it could still be another issue. I am not that familiar with SCSI set ups.

Cheers

Toby