IO-Playback stutters on viewer but not canvas

Hi:

This has been a sort of ongoing problem with my Io system (which we've had for five years or so), mostly an annoyance but I'd like to know what's going on.

When I play back a Quicktime (8-bit uncompressed, Photo JPEG, etc.) from the viewer, I get a fair amount of stuttering when viewing the video through the Io on an NTSC monitor (Video Playback set to ProIO-NTSC US-Component YPbPR Betacam 525-8-Bit 422(720x486). It's like very brief freezes every couple of seconds, especially with fast motion but sometimes with little motion. Sometimes the picture will freeze completely in NTSC, though the time marker is still moving. I don't believe the hard drive is dropping frames because the video will play back smoothly on the desktop if I set the video playback to None. I don't have any other Firewire devices connected. The video is playing from a fast internal SATA drive or array.

However, if I make a sequence with the appropriate codec and drop the Quicktime into that, that same Quicktime will play smoothly from the canvas through the Io into my NTSC monitor.

I'm using FCP 5.1.4 in OS 10.5.3 on a Mac Pro. I had similar issues on a G5 Mac running OS 10.4.

Any thoughts?

Thank you.

Shawn Marshall
Marshall Arts Motion Graphics