Multiclip troubles - continually dropping frames

Hi All-

I'm having a new problem editing multiclips. When playing back the
timeline, I'm getting the Dropped Frames warning, with the red text that
says "RT Extreme has determined these dropped frames were caused by slow
disks. Please try:
-Increasing the speed of your disks.
- Decreasing the number of RT layers.
- Limiting your RT bandwidth in User Preferences."

What's confusing is that this used to work fine. Here's the specs:

2 x 3 GHz Quad Core Intel Xeon (Late Summer 2007 era model)
OSX 10.5.7
16 GB RAM
4 x 750 GB Hard drives - came with system - 3 Gigabit drives
Injesting material from HDV via AJA IoHD to Prores 422 (not HQ)
3 clips in the multiclip, each at 1280x720, 59.94, ProRes 422, each around
16 MB/sec.
FCP 6.0.6
OSX 10.5.8

When I play one at a time, they are fine. It's only when I start to play
more than one that I get this problem. That said, since we've been using
ProRes, I've NEVER been able to play all three at 100% quality, I always
have to take the RT quality menu down to QUARTER frame rate. But, again,
that used to work, but doesn't any longer.

Without multiclip, I have to edit these manually, using three tracks, and
that's a pain!

I don't know that the problem is limited to multiclip playback, though. I
have noticed that I'm dropping more frames than I used to. Even after a
fresh restart... I'm still running into problems when playing back anything
that's not a "blue" render bar. (In other words, if it's "green", I will
usually have to render it before I can play it, or it will drop frames and
stop.) So I'm not so sure this is a problem with Multiclip, as just a problem with my machine being able to play anything that doesn't have a fully rendered bar at the top.

Any ideas on what I may have inadvertently changed? Thanks for any help you
can give.

-Carl Sundermann