I have a suped-up G4 running a RocketRAID PCI card. Three Hard drives are RAIDed in a stripped RAID 0 array. Disk tests get me close to 170megabytes/sec. That puts me solidly into HD Uncompressed 8 bit territory. Test videos I've used playback through Quicktime at full frame-rate and Activity monitor shows stable reading performace at the 90megabytes/sec that 8 bit runs at. BUT, when I take that same movie file and bring it into Final Cut Studio the same video will not playback without stuttering either out of the timeline with correct settings or in the canvas window. What gives?

Might it have something to do with your video card? I seem to remember having similar problems with everything below an ati 9800.